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.

TODO.md

@@ -9,17 +9,6 @@ Actionable task list for this project. This file is the single, central place fo
 - When all steps in a version section are done: write a `CHANGELOG.md` entry, then remove the version section from this file.
 - Release gate: a version is tagged only on the owner's explicit go-ahead once its release checks pass, not automatically when the checkboxes close.
 
-## 0.2.3 — Camera clipping fixes (patch)
-
-### Patch tasks - active - session-5598ce7e-27e3-4c64-bb49-c04a4a31ee58 (branch `camera-clip-0.2.3`)
-
-- [A] Fix remaining camera clipping cases in `0.2.x`. **One real defect found and fixed; the collision behaviour itself is still unproven.**
-  - **Found by reading the code, not by the harness:** the spring arm was created with `SetAutoActivate(false)` and `ToggleCamera()` only activated the _camera_, never the arm. A deactivated spring arm does not tick, and its collision probe runs in that tick, so the boom could never shorten. `ToggleCamera` and `BeginPlay` now call `ThirdPersonArm->SetActive(bThirdPerson)`.
-  - **The measurement that was supposed to prove it does not work yet.** `-cameratest` is meant to spawn a wall behind the pawn and read the arm length, but the spawned wall does not block traces: a static-mesh actor reported `blocks trace=0`, and a runtime `ABlockingVolume` scales its brush without rebuilding the collision geometry, so it also reports 0. Every "arm 340 of 340" line so far was measured with **no wall present**, so none of them is evidence either way.
-  - **Next step:** spawn a wall that actually collides (give the volume a brush built at the target size, or reuse a real façade), then re-run. Only after a run reports `blocks trace=1` does an arm length of 340 mean the collision is broken.
-  - **Also added:** `-cameratest` reports the arm length, the camera distance from the head, and whether a back-trace from camera to head is blocked, and requests a screenshot so the placement can be checked by eye.
-- [ ] Tag `v0.2.3` and write `CHANGELOG.md` entry; then remove this section.
-
 ## 0.3.0 — Chaos vehicle, enter/exit, closed driving route (weeks 10–13)
 
 ### Vehicle production