diff --git a/.gitignore b/.gitignore
index a2d7ca1..5479e54 100644
--- a/.gitignore
+++ b/.gitignore
@@ -9,3 +9,5 @@ calibration/runs/
 tools/drivers/
 tools/obs-spout2-plugin/
 tools/src/
+target/
+native/**/target/
diff --git a/docs/implementation-plan.md b/docs/implementation-plan.md
index c3d6336..6fae140 100644
--- a/docs/implementation-plan.md
+++ b/docs/implementation-plan.md
@@ -5,6 +5,7 @@
 - Perfect Machine contract:
   - `docs/perfect-machine.md` defines the target native machine: one five-second spatiotemporal reservoir, Aquarium-owned dense visual fusion/material/brush rendering, Faust-owned hot audio DSP, OBS-owned broadcast controls, and LocalCastBridge-owned calibration/config/status.
   - `config/perfect-machine.example.json` declares the contract shape for six cameras, six microphones, reservoir rings, native authorities, outputs, and the demotion of bridge scripts to tooling.
+  - `native/reservoir` is the first native Rust crate. It implements shared-edge five-second reservoir rings and proves that all rings expire against the newest sample, not private per-source clocks.
 - Repo-local persistence machinery.
 - First architecture map.
 - Example config for one video source plus two audio sources.
@@ -74,7 +75,7 @@
 
 ## Next
 
-1. Implement the native reservoir ABI declared by `config/perfect-machine.example.json`.
+1. Add a C ABI and Aquarium-facing binding around `native/reservoir`.
 2. Move camera ingest into native capture workers that append `camera_frame` samples to the reservoir.
 3. Move feature extraction, flow, cross-view matching, LOD reconciliation, and material fitting into Aquarium GPU compute.
 4. Move mic alignment, voice separation, room suppression, Ambisonic/HOA spatialization, and stem generation into Faust/native DSP.
diff --git a/docs/perfect-machine.md b/docs/perfect-machine.md
index a768302..ac334ae 100644
--- a/docs/perfect-machine.md
+++ b/docs/perfect-machine.md
@@ -147,3 +147,16 @@ Keep these as tooling:
 6. Let Faust consume reservoir audio blocks and source controls directly.
 7. Keep the old bridge only as a comparison and fallback surface.
 
+## Native Work Started
+
+`native/reservoir` is the first native crate. It implements the shared-edge
+five-second reservoir invariant:
+
+- append samples into named rings;
+- advance the reservoir edge from the newest sample;
+- evict every ring against the same edge;
+- query the latest valid sample per sensor.
+
+This is intentionally below Aquarium and Faust. The crate owns retention and
+sample-window semantics; Aquarium owns GPU interpretation and rendering, Faust
+owns DSP interpretation.
diff --git a/native/README.md b/native/README.md
new file mode 100644
index 0000000..5bab682
--- /dev/null
+++ b/native/README.md
@@ -0,0 +1,13 @@
+# Native Runtime
+
+This directory is for the hot path.
+
+LocalCastBridge Python remains useful for calibration, launch, status, contract
+tests, and offline analysis. It does not own dense live fusion or DSP.
+
+## Crates
+
+- `reservoir`: the first native five-second spatiotemporal reservoir core. It
+  owns shared-edge retention and typed sample rings. Aquarium/Faust integration
+  should build on this invariant instead of copying the deadline bridge cache.
+
diff --git a/native/reservoir/Cargo.lock b/native/reservoir/Cargo.lock
new file mode 100644
index 0000000..8ce6767
--- /dev/null
+++ b/native/reservoir/Cargo.lock
@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "localcast-reservoir"
+version = "0.1.0"
diff --git a/native/reservoir/Cargo.toml b/native/reservoir/Cargo.toml
new file mode 100644
index 0000000..13cd157
--- /dev/null
+++ b/native/reservoir/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "localcast-reservoir"
+version = "0.1.0"
+edition = "2024"
+description = "Native five-second spatiotemporal reservoir core for LocalCastBridge"
+
+[lib]
+crate-type = ["rlib"]
+
+[dependencies]
diff --git a/native/reservoir/src/lib.rs b/native/reservoir/src/lib.rs
new file mode 100644
index 0000000..2a7b204
--- /dev/null
+++ b/native/reservoir/src/lib.rs
@@ -0,0 +1,211 @@
+use std::collections::{BTreeMap, VecDeque};
+
+pub const DEFAULT_RESERVOIR_NS: u64 = 5_000_000_000;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum RingKind {
+    CameraFrame,
+    CameraFeature,
+    SceneRay,
+    SurfaceClaim,
+    MaterialClaim,
+    AudioBlock,
+    PhaseClaim,
+    EventClaim,
+    RenderPacket,
+}
+
+#[derive(Debug, Clone, PartialEq)]
+pub struct Sample<T> {
+    pub sensor_id: String,
+    pub timestamp_ns: u64,
+    pub arrival_ns: u64,
+    pub sequence: u64,
+    pub payload: T,
+}
+
+impl<T> Sample<T> {
+    pub fn new(
+        sensor_id: impl Into<String>,
+        timestamp_ns: u64,
+        arrival_ns: u64,
+        sequence: u64,
+        payload: T,
+    ) -> Self {
+        Self {
+            sensor_id: sensor_id.into(),
+            timestamp_ns,
+            arrival_ns,
+            sequence,
+            payload,
+        }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct ReservoirRing<T> {
+    duration_ns: u64,
+    edge_ns: u64,
+    samples: VecDeque<Sample<T>>,
+}
+
+impl<T> ReservoirRing<T> {
+    pub fn new(duration_ns: u64) -> Self {
+        Self {
+            duration_ns,
+            edge_ns: 0,
+            samples: VecDeque::new(),
+        }
+    }
+
+    pub fn push(&mut self, sample: Sample<T>) {
+        self.edge_ns = self.edge_ns.max(sample.timestamp_ns);
+        self.samples.push_back(sample);
+        self.evict_expired();
+    }
+
+    pub fn set_edge(&mut self, edge_ns: u64) {
+        self.edge_ns = self.edge_ns.max(edge_ns);
+        self.evict_expired();
+    }
+
+    pub fn edge_ns(&self) -> u64 {
+        self.edge_ns
+    }
+
+    pub fn window_start_ns(&self) -> u64 {
+        self.edge_ns.saturating_sub(self.duration_ns)
+    }
+
+    pub fn len(&self) -> usize {
+        self.samples.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.samples.is_empty()
+    }
+
+    pub fn iter(&self) -> impl Iterator<Item = &Sample<T>> {
+        self.samples.iter()
+    }
+
+    pub fn latest_for_sensor(&self, sensor_id: &str) -> Option<&Sample<T>> {
+        self.samples
+            .iter()
+            .rev()
+            .find(|sample| sample.sensor_id == sensor_id)
+    }
+
+    fn evict_expired(&mut self) {
+        let start_ns = self.window_start_ns();
+        self.samples
+            .retain(|sample| sample.timestamp_ns >= start_ns && sample.timestamp_ns <= self.edge_ns);
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct Reservoir<T> {
+    duration_ns: u64,
+    edge_ns: u64,
+    rings: BTreeMap<RingKind, ReservoirRing<T>>,
+}
+
+impl<T> Reservoir<T> {
+    pub fn new(duration_ns: u64) -> Self {
+        Self {
+            duration_ns,
+            edge_ns: 0,
+            rings: BTreeMap::new(),
+        }
+    }
+
+    pub fn push(&mut self, kind: RingKind, sample: Sample<T>) {
+        self.edge_ns = self.edge_ns.max(sample.timestamp_ns);
+        for ring in self.rings.values_mut() {
+            ring.set_edge(self.edge_ns);
+        }
+        let ring = self
+            .rings
+            .entry(kind)
+            .or_insert_with(|| ReservoirRing::new(self.duration_ns));
+        ring.push(sample);
+        ring.set_edge(self.edge_ns);
+    }
+
+    pub fn set_edge(&mut self, edge_ns: u64) {
+        self.edge_ns = self.edge_ns.max(edge_ns);
+        for ring in self.rings.values_mut() {
+            ring.set_edge(self.edge_ns);
+        }
+    }
+
+    pub fn ring(&self, kind: RingKind) -> Option<&ReservoirRing<T>> {
+        self.rings.get(&kind)
+    }
+
+    pub fn edge_ns(&self) -> u64 {
+        self.edge_ns
+    }
+
+    pub fn window_start_ns(&self) -> u64 {
+        self.edge_ns.saturating_sub(self.duration_ns)
+    }
+}
+
+impl<T> Default for Reservoir<T> {
+    fn default() -> Self {
+        Self::new(DEFAULT_RESERVOIR_NS)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn ring_expires_from_newest_shared_edge() {
+        let mut ring = ReservoirRing::new(DEFAULT_RESERVOIR_NS);
+        ring.push(Sample::new("cam-a", 1_000_000_000, 1_000_000_010, 0, 1));
+        ring.push(Sample::new("cam-b", 4_000_000_000, 4_000_000_010, 0, 2));
+        ring.push(Sample::new("cam-a", 7_100_000_000, 7_100_000_010, 1, 3));
+
+        let timestamps = ring
+            .iter()
+            .map(|sample| sample.timestamp_ns)
+            .collect::<Vec<_>>();
+
+        assert_eq!(vec![4_000_000_000, 7_100_000_000], timestamps);
+        assert_eq!(2_100_000_000, ring.window_start_ns());
+    }
+
+    #[test]
+    fn reservoir_edge_expires_all_rings() {
+        let mut reservoir = Reservoir::default();
+        reservoir.push(
+            RingKind::CameraFrame,
+            Sample::new("cam-a", 1_000_000_000, 1_000_000_010, 0, "old-camera"),
+        );
+        reservoir.push(
+            RingKind::AudioBlock,
+            Sample::new("audio", 7_000_000_001, 7_000_000_020, 0, "audio-edge"),
+        );
+
+        let camera_ring = reservoir.ring(RingKind::CameraFrame).unwrap();
+
+        assert!(camera_ring.is_empty());
+        assert_eq!(2_000_000_001, reservoir.window_start_ns());
+    }
+
+    #[test]
+    fn latest_for_sensor_ignores_other_sources() {
+        let mut ring = ReservoirRing::new(DEFAULT_RESERVOIR_NS);
+        ring.push(Sample::new("cam-a", 10, 11, 0, "a0"));
+        ring.push(Sample::new("cam-b", 12, 13, 0, "b0"));
+        ring.push(Sample::new("cam-a", 14, 15, 1, "a1"));
+
+        let latest = ring.latest_for_sensor("cam-a").unwrap();
+
+        assert_eq!("a1", latest.payload);
+        assert_eq!(1, latest.sequence);
+    }
+}
diff --git a/notes/current-system-map.md b/notes/current-system-map.md
index 67dfd6c..eccef36 100644
--- a/notes/current-system-map.md
+++ b/notes/current-system-map.md
@@ -27,6 +27,7 @@ Ownership:
 - Faust owns hot audio DSP and program stem generation.
 - OBS owns broadcast controls.
 - Python remains tooling, not the stream hot path.
+- `native/reservoir` is the first native crate. It owns shared-edge five-second sample retention for typed rings and is the seed of the Aquarium/Faust runtime boundary.
 
 Invariant: the five-second spatiotemporal reservoir is the live authority. Producers append, optimizers refine, Aquarium/Faust sample. No private history outlives the reservoir.
 
diff --git a/state/map.yaml b/state/map.yaml
index 8679004..3e6bdd7 100644
--- a/state/map.yaml
+++ b/state/map.yaml
@@ -33,6 +33,8 @@ invariants:
   - Local secrets remain untracked.
 
 accepted_design:
+  - `docs/perfect-machine.md` and `config/perfect-machine.example.json` define the native target: one five-second spatiotemporal reservoir, Aquarium GPU fusion/rendering, Faust DSP, OBS output controls, and LocalCastBridge tooling.
+  - `native/reservoir` is the first native Rust crate. It implements shared-edge reservoir rings so all sample types expire from the newest live sample rather than private source histories.
   - `config/localcast.example.json` defines receiver address, base port, video encode settings, and audio sources.
   - `scripts/sender-discover.ps1` lists FFmpeg devices, protocols, and encoders.
   - `scripts/sender-start.ps1` launches one FFmpeg process per enabled source.