fix(camera): prove the boom shortens, and record how to repeat it

With the spring arm activated the collision now works: a 2 m wall spawned 4 m behind the
pawn shortens the boom from 340 cm to 288 cm - 12 cm clear of the wall, matching the probe
size. -cameratest logs it, writes a placement screenshot, and the measurement is recorded
as a release check for 0.2.3.

Two traps documented because each one made a working mechanism look broken:
  - a spawned static-mesh actor has no collision unless it is created deferred and given
    its mesh and collision before FinishSpawning (three runs reported blocks trace=0)
  - TargetArmLength is not rewritten when the boom shortens, so reading the property
    reports the requested length; the camera's distance from the head is the measurement

Writes the 0.2.3 changelog entry and prunes the section from TODO.md.

CHANGELOG.md

@@ -8,6 +8,28 @@ Format: [Keep a Changelog](https://keepachangelog.com/) style, versions follow [
 
 Nothing yet.
 
+## [0.2.3] — 2026-11-09
+
+Camera collision worked in the code and never in the game.
+
+### Fixed
+
+- **The third-person boom never shortened.** The spring arm was created with `SetAutoActivate(false)` and `ToggleCamera()` activated only the camera. A deactivated spring arm does not tick, and its collision probe runs in that tick, so the boom could not shorten no matter what stood behind the pawn. Both `ToggleCamera` and `BeginPlay` now activate the arm with the rig.
+
+### Added
+
+- **`-cameratest`** — backs the pawn against a spawned 2 m wall, waits for the boom, and reports the shortening, whether the boom ends inside geometry, and a back-trace from camera to head. It writes `review-demo-camera-collision.png` so the placement can be checked by eye.
+
+### Verified
+
+- With a wall 4 m behind the pawn the boom shortens from 340 cm to **288 cm** — 12 cm clear of the wall, matching the probe size. Recorded as a release check.
+- 8 review shots verify; 95 tool tests pass.
+
+### Notes for future work
+
+- **A spawned static-mesh actor has no collision unless it is deferred.** Setting the mesh and collision profile after `SpawnActor` produced a wall that blocked nothing (`blocks trace=0`) on three consecutive attempts. `SpawnActorDeferred` → set mesh and collision → `FinishSpawning` builds the physics state correctly.
+- **`TargetArmLength` is not rewritten when the boom shortens.** The spring arm applies the shortened length to the camera, so the measurement must be the camera's distance from the head; reading the property reports the requested length and looks like a failure.
+
 ## [0.2.2] — 2026-11-09
 
 The character becomes a character: clothed, haired, and measured against a generated blueprint instead of judged by eye.