Validation: check convex-hull vertex count per collider (engine budget check)

[Weisl]

Context

check_triangle_count in validation/checks.py checks whole-collider triangle count against a configurable threshold, but the actual limits engines enforce on convex colliders are per-hull vertex counts, not triangle counts:

• Unity: convex MeshCollider is capped at 255 vertices per hull; exceeding it silently produces a simplified hull.
• Unreal: practical/recommended cap is roughly 8-32 verts per convex hull; exceeding it gets re-simplified by the importer in engine-chosen ways.

Neither is currently caught by validation — a collider can pass all existing checks and still blow an engine's real import-time budget.

Proposal

Add a new validation check (alongside the existing checks in validation/checks.py, e.g. check_convex_hull_vertex_count) that flags colliders whose convex hull vertex count exceeds a configurable threshold, following the same pattern as check_too_many_colliders / check_triangle_count (numeric threshold in Preferences > Validation tab, surfaced in the Validation results UIList).

Out of scope for a first pass: auto-deriving the default threshold from the active naming preset (Unity-default -> 255, UE-default -> ~32) - worth a follow-up once the base check exists, not a blocker for it.

Acceptance criteria

• New check appears in Validation panel results with a clear message (e.g. "N verts (limit M)"), same style as existing checks.
• Threshold is configurable in Preferences, matching the existing validation threshold pattern.
• Test coverage alongside existing validation check tests.