chore: strip throwaway dense-town FPS profiling apparatus (plan Task 5)

The FPS deep-dive landed (dense Arwic 75 -> ~165 fps via the cell-object
batching + cell-particle consolidation, both already committed). Remove the
throwaway diagnostic apparatus now that it has served its purpose:

- delete FrameProfiler.cs (whole-frame TimeElapsed + [PASS-GPU] glFinish +
  [CPU-PHASE]/[GPU-PHASE] timers + the =1/=2 ACDREAM_FPS_PROF modes)
- GameWindow: _fpsProf/_frameProfiler/_msaaSamples fields, the BeginFrame/
  EndFrame/MarkUpdateStart hooks, the terrain glFinish, and the landscape
  sub-phase LsMark instrumentation
- RetailPViewRenderer: the DrawInside per-phase Phase()/MarkGpu markers
- ParticleRenderer / PortalDepthMaskRenderer / EnvCellRenderer: the per-pass
  glFinish brackets
- delete DegradeCoverageProbeTests.cs (the dead distance-degrade probe)

KEPT (the real fixes): RetailPViewRenderer cell-object batching + consolidated
cell-particle pass; EnvCellRenderer.CellHasTransparent. Build + full test suite
green (468 App incl. pview replay tests; 1566 Core; 317 Net; 425 UI).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/ParticleRenderer.cs

@@ -123,11 +123,6 @@ public sealed unsafe class ParticleRenderer : IDisposable
         ParticleRenderPass renderPass = ParticleRenderPass.Scene,
         Func<AcDream.Core.Vfx.ParticleEmitter, bool>? emitterFilter = null)
     {
-        // THROWAWAY FPS apparatus: glFinish-bracket particle GPU (ACDREAM_FPS_PROF).
-        System.Diagnostics.Stopwatch? _pSw = null;
-        if (AcDream.App.Rendering.FrameProfiler.PassGpuEnabled) { _gl.Finish(); _pSw = System.Diagnostics.Stopwatch.StartNew(); }
-        try
-        {
         if (particles is null || camera is null)
             return;
 
@@ -173,15 +168,6 @@ public sealed unsafe class ParticleRenderer : IDisposable
         _gl.BindVertexArray(0);
         _gl.DepthMask(true);
         _gl.Disable(EnableCap.Blend);
-        }
-        finally
-        {
-            if (_pSw is not null)
-            {
-                _gl.Finish();
-                AcDream.App.Rendering.FrameProfiler.AddRendererGpu("particles", _pSw.Elapsed.TotalMilliseconds);
-            }
-        }
     }
 
     private List<ParticleDraw> BuildDrawList(