[codex] optimize mesh submesh validation for static scenes

[kitwongpixel]

What changed

• Avoid calling Geometry.validate() on every Mesh.subMeshes access unless the geometry is a PrimitiveBase that can actually rebuild itself lazily.
• Keep the existing sub-mesh list behavior intact for normal geometries.

Why

• Issue 100$ Bounty. Huge performance difference on html5 with multiple meshes and merged in a single mesh. #26 reports a large HTML5 performance gap between many separate meshes and a merged mesh.
• This removes a per-frame validation call from the hot path for static/loaded meshes, which should reduce overhead in large scenes.

Impact

• Static meshes and ordinary loaded geometries avoid a redundant validate check every render loop iteration.
• Primitive geometries still validate as needed, so primitives keep their existing lazy rebuild behavior.

Validation

• git diff --check
• Syntax reviewed manually in away3d/entities/Mesh.hx
• I could not run Haxe/OpenFL compilation in this container because haxe is not installed here.