Lodur/docs/first-slice.md

First slice: a PD-muscle ragdoll on Ymir

Bad Day Simulator ladder rung 1: a body that stands, falls and rests deterministically, read through Ymir's structure-of-arrays world. Nothing above layer 5 exists yet.

What is built

• Lodur.Core (netstandard2.1, no references): Quat, MuscleGains, PdMuscle.Torque. A PD law on one joint: the child's rotation relative to the parent and their relative angular velocity in, a torque out. Strength is a multiplier on every gain; zero is limp and returns exactly zero. Swing (joint frame x, y) and twist (z) have separate gains.
• Lodur.Body.Ymir (net8.0, references Ymir.Box3D): HumanoidRagdoll builds 13 bodies and 12 joints in a caller-owned Box3DSoaWorld: capsule limbs, box pelvis, torso and feet, a sphere head, ball joints at spine, neck, shoulders and hips, hinges at elbows, knees and ankles. Drive(strengths, targets) writes each muscle's torque onto the world's torque lanes (equal and opposite on child and parent). The strength array and the target array belong to the caller; nothing is copied or retained, and a tick allocates nothing (tested).
• Tests (tests/Lodur.Tests): the PD law's behaviour; stand, fall, rest; per-muscle strength; torque conservation; fractional strength scaling; no allocation; same seed, same state hash.

Measured (Linux x64, Debug native build, one machine)

• At 120 Hz, with full strength and a random one-step shove from the seed (up to 30 N m per axis on pelvis and torso), the ragdoll holds head height 1.73 m (initial 1.74) through 1.5 s for seeds 1, 2, 3 (tested) and through about 2 s in the exploratory runs. It then topples: there is no balance controller, so a pose-holding PD ragdoll is an inverted pendulum with soft ankles.
• With zero strength it falls (head under 0.6 m within 1 s) and its trunk comes to rest within 6 s for seeds 1, 2, 3 (trunk under 0.05 m/s and 0.2 rad/s). Extremities can keep rocking: a head resting on an arm chattered at up to about 3.5 rad/s for 10 s in one seed. The test bounds it (under 0.6 m/s, 6 rad/s); it does not claim every body is still.
• Same seed gives the same FNV-1a hash of every body and joint lane at 14 checkpoints, in one process. A different seed, or one strength element changed, gives a different hash.

Not measured, or limits found

• Cross-platform bit-identity: unclaimed. Windows build and Unity consumption: unrun. One machine, Debug native build only; no Release run, no run in a second process.
• Explicit PD through torque lanes limits stiffness and damping: damping times the step must stay under the inertia it acts on. Hips and spine went unstable at damping 15 and 20 and are set to 3 and 4; the gains are tuned by running, not derived. Stiff ankles (what balance needs) need joint springs or motors inside the solver, which Ymir's SoA world does not expose yet.
• A bare sphere rolls on forever, so the head takes a rolling resistance (0.1).
• Hands, a rig licence question, impairment, contact facts (pain input), body removal: not built.