Signatures: every light signature names its channel and item through Entity.SetSignature

VisibilitySources holds Signature (channel, strength, item); the eight direct writes
go through the one writer; Visibility stays the sum; SignatureOn(channel) splits it.
VisibilityData gains Key(3) Channel. No number changes. Not yet built or tested at this
commit (verification follows).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Assets/Scripts/ServerShared/Sensing.cs

@@ -0,0 +1,30 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+// Light signatures split by channel (ruling visibility-per-capability-reveal): an observer learns what a ship
+// is by which channel carries the light. Append-only: values are serialized in VisibilityData.
+public enum SignatureChannel
+{
+    Thermal,
+    Reflection,
+    DrivePlume,
+    LaunchFlash,
+    Emission
+}
+
+public readonly struct Signature
+{
+    public readonly SignatureChannel Channel;
+    public readonly float Strength;
+    public readonly EquippedItem Item;
+
+    public Signature(SignatureChannel channel, float strength, EquippedItem item)
+    {
+        Channel = channel;
+        Strength = strength;
+        Item = item;
+    }
+
+    public Signature WithStrength(float strength) => new Signature(Channel, strength, Item);
+}