perf(goal): merge goal visuals into one ArrayMesh

Goal._build_visuals() built 10 MeshInstance3D nodes across 4 materials
(1 pocket + 1 net + 4 bezel-ring + 4 rim-ring boxes) per goal. Replaced
with a single MeshInstance3D wrapping one ArrayMesh with 4 SurfaceTool-
committed surfaces (pocket, net, bezel, rim), one material per surface
via surface_set_material — 10 nodes down to 1, same 4 materials.

Kept 4 surfaces rather than collapsing further: the rim is a tuned
team-tinted emitter, the net carries its own discard shader, and the
pocket/bezel differ in albedo/metallic/roughness. Merging those into a
shared material would be a visible regression, not a free win.

Added a local box-to-SurfaceTool helper (6 quads via arena_boundary.gd's
_add_quad/_add_tri winding-correction trick, copied in rather than
shared since that file's geometry is collision-adjacent). The pocket's
old cull_mode = CULL_FRONT trick is replaced by emitting its geometry
with inverted winding; the net's cull_front stays material-driven since
goal_net.gdshader's own render_mode depends on that winding convention.

Verified in the editor: both team-tinted goals render an intact pocket,
net, bezel and glowing rim with no backface/winding artifacts, and the
headless free_play smoke test still runs clean.

TODO.md

@@ -26,7 +26,6 @@ Bugs found in an adversarial review. None are gameplay- or physics-affecting, so
 ## Performance
 
 - [ ] 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.
 - [ ] 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.