perf(arena): cache the active camera lookup in ArenaBoundary

_process() called get_viewport().get_camera_3d() every frame to drive
the containment field's camera-side fade. Cache it the same way
ship_camera.gd caches the ball, revalidating with is_instance_valid
since exactly one camera rig is spawned per game-mode run today.

TODO.md

@@ -28,7 +28,6 @@ Bugs found in an adversarial review. None are gameplay- or physics-affecting, so
 - [ ] Shared per-team materials instead of `Ship._apply_team_color()` allocating a fresh `StandardMaterial3D` and assigning it as `material_override` (and running at least twice per ship — once from `_ready`, once from the `team` setter). Removes the allocation and lets same-team ships batch.
 - [ ] Merge each goal's visuals into one `ArrayMesh` with a hull surface and a net surface — currently 11 `MeshInstance3D`s and 4 materials per goal, ~22 draw calls for a static prop. `arena_boundary.gd:333` already demonstrates the `SurfaceTool` technique in this codebase.
 - [ ] Merge the four non-tinted ship meshes (Hull/Canopy/EngineGlowL/R) into one — 6 draw calls per ship down to 3. Irrelevant at 1v1; 36 calls before VFX at 3v3.
-- [ ] Cache the camera in `ArenaBoundary._process` instead of a `get_viewport().get_camera_3d()` tree lookup every frame (`ship_camera.gd` already caches the ball this way).
 - [ ] Name collision layers in `project.godot` and assign them — nothing configures `collision_layer`/`collision_mask` today, so every body tests against every other.
 - [ ] Measure `nebula_dust.gdshader`'s per-fragment depth-texture sample across 500 large soft billboards before adding more particle work.
 - [ ] Bake `ArenaBoundary`'s ~160 runtime-generated `CollisionShape3D` nodes into the scene. Costs a load hitch on every arena entry and repeats in every parallel headless training env. **Blocked on the trained-bot decision below** — `arena_boundary.gd:235` notes this geometry is what the shipped policies were fitted against, so the bake must be verified byte-identical.