perf(ship): share per-team accent material instead of allocating per ship

_apply_team_color() allocated a fresh StandardMaterial3D on every call, and
ran at least twice per ship (once from _ready at the default team, once from
the team setter when the game mode assigns the real team). Cache one
StandardMaterial3D per team in a static dict on Ship and reuse it across
every ship on that team.

TODO.md

@@ -25,7 +25,6 @@ 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 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.