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>

docs/plans/2026-04-11-roadmap.md

@@ -687,22 +687,26 @@ for our deletions/additions; merge upstream `master` periodically.
   manifest at higher radius. Spec acceptance criterion #5 was wrong;
   amended via `docs/plans/2026-05-09-phase-n5b-perf-baseline.md`. Plan
   archived at `docs/superpowers/plans/2026-05-09-phase-n5b-terrain-modern.md`.
-- **N.6 — Perf polish.** **Planned (post-A.5 polish takes priority).**
-  Builds on N.5 + N.5b. Legacy renderer retirement was pulled forward
-  into N.5 ship amendment — `InstancedMeshRenderer`, `StaticMeshRenderer`,
-  `WbFoundationFlag` are gone — and the terrain legacy renderer
-  (`TerrainChunkRenderer` + `TerrainRenderer` + `terrain.vert/.frag`)
-  retired in N.5b. N.6 scope: WB atlas adoption for memory savings
-  on shared content, persistent-mapped buffers if `glBufferData` shows
-  up in profiling (the modern terrain path's per-frame DEIC `BufferSubData`
-  is a candidate), GPU-side culling via compute pre-pass (eliminates
-  the per-frame slot walk + DEIC build entirely), GL_TIME_ELAPSED query
-  double-buffering (deferred from N.5 — diagnostic shows `gpu_us=0/0`
-  under `ACDREAM_WB_DIAG=1`), direct higher-radius perf comparison (A.5
-  has now landed — modern's architectural wins are measurable), retire the
-  legacy `Texture2D`/`sampler2D` path in `TextureCache` (currently kept
-  for Sky + Debug + particle paths now that Terrain has migrated).
-  Plan + spec written when work begins. **Estimate: 1-2 weeks.**
+- **N.6 slice 1 — GPU timing fix + radius=12 perf baseline.** **SHIPPED 2026-05-11.**
+  Fixed the gpu_us double-buffering bug in `WbDrawDispatcher` (ring-of-3
+  query slots, read-before-overwrite, vendor-neutral across AMD/NVIDIA/Intel
+  desktop GL). Added env-gated surface-format histogram dump in `TextureCache`
+  for atlas-opportunity audit. Captured authoritative baseline at Holtburg
+  radii 4 / 8 / 12 (standstill + walking) with the now-working `gpu_us`
+  diagnostic. Plan + spec at `docs/superpowers/{specs,plans}/2026-05-11-phase-n6-slice1-*.md`.
+  Baseline numbers + next-phase recommendation at
+  [docs/plans/2026-05-11-phase-n6-perf-baseline.md](2026-05-11-phase-n6-perf-baseline.md).
+- **N.6 slice 2 — Perf polish cleanup.** **Planned — deferred until after C.1.5
+  (PES emitter wiring) per the baseline doc's recommendation.** Builds on
+  slice 1's measurement. Scope: retire the legacy `Texture2D`/`sampler2D` path
+  in `TextureCache` (currently kept for Sky + Debug + particle paths now that
+  Terrain has migrated); delete orphan `mesh.frag` (verify zero callers post-N.5
+  amendment); decide bindless-everywhere vs legacy-island for the remaining
+  `sampler2D` consumers. **Dropped from slice 2 scope per baseline data**:
+  WB atlas adoption and persistent-mapped buffers — both target GPU/sampler
+  throughput but the baseline shows GPU is wildly under-utilized (max gpu_us
+  p95 ~600 µs vs 16,600 µs frame budget). Slice 2 reduces to a ~1-day cleanup.
+  Plan + spec written when work begins. **Estimate: ~1 day once C.1.5 lands.**
 - **N.7 — EnvCells / dungeons.** Replace EnvCell rendering with WB's
   `EnvCellRenderManager` + `PortalRenderManager` on top of N.4's
   foundation. **Estimate: 1-2 weeks** (was 2-3 — naturally smaller now