feat: third-person camera toggle (V) + fix first-person showing inside the model

The first-person head-shrink (SkeletonModifier3D) left the camera looking at
the inside of the neck/torso/hair. Replaced it: the owner's model now renders
shadows-only in first person (clean FPS view, still fully animated and visible
to others and in shadows).

Added a third-person toggle (V / toggle_camera_view) so you can actually see
your own animated model — the easiest way to verify a new skin's animations:
- over-the-shoulder SpringArm3D camera (wall-collision aware) on the local
  player; press V to swap, press again to return
- firing still uses the first-person camera, so aim is unchanged
- reveals the owner's model via set_owner_visible() on both SkinnedPlayerModel
  and HumanoidModel; hides the first-person weapon viewmodel while in TP
- death forces first person so the toggle can't fight the death cam

Smoke test extended to cover the toggle (camera boom created, becomes current,
round-trips) — 28/28 checks pass for both color and GLB skins.

Co-Authored-By: Claude Fable 5 <[email protected]>

docs/3D_ASSET_PIPELINE.md

@@ -100,11 +100,16 @@ animates in every movement state.
 - **`SkinnedPlayerModel`** loads the GLB at runtime, maps canonical clip
   names, sets loop modes, blends between clips (0.15 s), and scales
   locomotion playback speed to actual movement speed.
-- **First person (owner):** full body visible and animated; the head bone is
-  shrunk via a `SkeletonModifier3D` so you never see your own face — you DO
-  see your legs, arms and shadow.
-- **Third person (everyone else):** the full model, driven by the synced
-  movement state, with the current weapon attached to the right-hand bone.
+- **First person (owner):** the model renders shadows-only for the owner (the
+  camera sits inside the head, so drawing the mesh would show its inside). It's
+  still fully animated and casts a real shadow; other players see the full body.
+- **Third person:** press **V** (`toggle_camera_view`) to swap to an
+  over-the-shoulder `SpringArm3D` camera and reveal your own animated model —
+  the easiest way to eyeball that a new skin's animations look right. Firing
+  still uses the first-person camera, so aim is unchanged. Press V again to
+  return to first person.
+- **Other players** always see the full third-person model, driven by the
+  synced movement state, with the current weapon attached to the hand bone.
 - **Licensing:** every Sketchfab download writes `<name>.license.json`.
   CC-BY models require crediting the author — keep these files and surface
   them in your credits screen before shipping.