hardening(render): Phase A8 — IndoorCellStencilPipeline robustness

Three small improvements from Task 5 code review:
- MarkAndPunch now enables DepthTest explicitly (was relying on
  GameWindow's startup enable; this makes the method self-contained).
- Uniform location fields marked readonly (set once in ctor).
- AllocateVbo gets a comment noting that mid-session reallocation is
  safe because the VAO bakes the VBO association at ConfigureVao time.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/IndoorCellStencilPipeline.cs

@@ -99,8 +99,8 @@ public sealed unsafe class IndoorCellStencilPipeline : IDisposable
     private readonly uint _vbo;
     private int _vboCapacityVerts;
     private int _lastVertexCount;
-    private int _uViewProjectionLoc;
-    private int _uWriteFarDepthLoc;
+    private readonly int _uViewProjectionLoc;
+    private readonly int _uWriteFarDepthLoc;
 
     public IndoorCellStencilPipeline(GL gl, string vertPath, string fragPath)
     {
@@ -153,6 +153,7 @@ public sealed unsafe class IndoorCellStencilPipeline : IDisposable
         if (_lastVertexCount == 0) return;
 
         _gl.Enable(EnableCap.StencilTest);
+        _gl.Enable(EnableCap.DepthTest);   // idempotent if already on; makes MarkAndPunch self-contained
         _gl.ClearStencil(0);
         _gl.Clear(ClearBufferMask.StencilBufferBit);
 
@@ -227,6 +228,9 @@ public sealed unsafe class IndoorCellStencilPipeline : IDisposable
         _gl.DeleteBuffer(_vbo);
     }
 
+    // Safe to call mid-session after ConfigureVao — the VAO bakes the
+    // VBO association at VertexAttribPointer time, so reallocating the
+    // VBO with new size does NOT require re-running ConfigureVao.
     private void AllocateVbo(int capacityVerts)
     {
         _vboCapacityVerts = capacityVerts;