docs(perf): Phase N.6 slice 1 — radius=12 baseline + surface dump path

Capture authoritative CPU+GPU dispatch numbers at Holtburg with the
gpu_us diagnostic now working (commit 25cb147). Three radii (4/8/12)
x two motion modes (standstill/walking) + a surface-format histogram
from ACDREAM_DUMP_SURFACES=1.

Adds env-gated one-shot dump path (TextureCache.TickSurfaceHistogramDumpIfEnabled,
called from GameWindow.OnRender) that fires once after both (a) frame
600 of the session AND (b) the upload-metadata dict reaches 100 entries
-- the cache-size gate prevents the dump from firing during pre-world
GUI ticks where OnRender spins at high rates but no scenery has streamed.
Output writes to %LOCALAPPDATA%\acdream\n6-surfaces.txt with a try/catch
around the I/O so disk-full / permission errors don't crash mid-measurement.

Baseline document at docs/plans/2026-05-11-phase-n6-perf-baseline.md
documents:
- CPU dominates GPU by 30-50x at every radius (strongly CPU-bound)
- GPU wildly under-utilized (max gpu_us p95 ~600us vs 16,600us frame budget)
- CPU scales superlinearly with N1 (Tier 1 cache wins on inner loop but
  not outer LB walk)
- Surface atlas opportunity high (59% of textures in top-3 triples) but
  win is memory-only since GPU isn't bottlenecked

Recommendation: C.1.5 (PES emitter wiring) next, then a reduced-scope
N.6 slice 2 (drop atlas + persistent-mapped buffers -- not justified by
the GPU under-utilization observed).

Roadmap entry amended to split N.6 into slice 1 (shipped) and slice 2
(planned, reduced scope, deferred until after C.1.5).

Spec: docs/superpowers/specs/2026-05-11-phase-n6-slice1-design.md.
Plan: docs/superpowers/plans/2026-05-11-phase-n6-slice1.md (Task 4).

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -6310,6 +6310,10 @@ public sealed class GameWindow : IDisposable
 
         _gl!.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);
 
+        // Phase N.6 slice 1: one-shot surface-format histogram dump under
+        // ACDREAM_DUMP_SURFACES=1. Zero cost when off.
+        _textureCache?.TickSurfaceHistogramDumpIfEnabled();
+
         // Phase N.4: drain WB pipeline queues (staged mesh data +
         // GL thread queue). Must happen before any draw work so that
         // resources uploaded this frame are available immediately.