docs(N.5b T10): roadmap + ISSUES + CLAUDE.md + perf baseline updates

Document Phase N.5b shipping (terrain on the modern rendering path via
Path C — `TerrainModernRenderer` mirrors WB's `TerrainRenderManager`
pattern but consumes acdream's `LandblockMesh.Build` so retail's
`FSplitNESW` formula stays in lockstep with physics + visual mesh).

Changes:

- `docs/plans/2026-04-11-roadmap.md` — add N.5b row to the Shipped
  table; promote N.5b's "Phases ahead" entry to ✓ SHIPPED with the
  Path C resolution + perf reality check; refresh N.6 scope to note
  Terrain has joined the modern path (legacy `Texture2D` retirement
  scope narrows to Sky + Debug); update top-of-doc Status line.

- `docs/ISSUES.md` — close issue #51 (WB terrain-split formula
  divergence). Move from OPEN to "Recently closed" with the Path C
  resolution: never adopted WB's formula; modern dispatcher uses
  retail's via `LandblockMesh.Build`. References `da56063` (the
  black-terrain fix that landed within the N.5b ship chain).

- `CLAUDE.md` — add `TerrainModernRenderer.cs` to the WB integration
  cribs list with the GL_INVALID_OPERATION caveat (use uvec2 +
  `sampler2DArray(handle)` constructor, NOT direct
  `uniform sampler2DArray` + `glProgramUniformHandleARB`). Update
  the "Currently in flight" preamble: N.6 builds on N.5 + N.5b;
  add an N.5b shipped paragraph linking the perf baseline doc.

- `docs/plans/2026-05-09-phase-n5b-perf-baseline.md` — new doc
  capturing the radius=5 Holtburg perf measurement (modern 6.4-7.0
  µs median vs legacy 1.5 µs — modern is ~4× SLOWER on CPU at
  radius=5). Documents the spec acceptance criterion #5 amendment,
  the architectural wins that DO hold (zero glBindTexture/frame,
  constant-cost dispatch as A.5 raises radius, per-LB frustum cull),
  and the three high-value gotchas surfaced during implementation.

User-memory updates (outside repo, not in this commit):
- `memory/project_phase_n5b_state.md` — full N.5b state file with
  the three gotchas captured.
- `memory/MEMORY.md` — index entry pointing at the state file.

Build: dotnet build green. No code changes in this commit.

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

CLAUDE.md

@@ -102,6 +102,14 @@ ourselves".
   eventually picks it up finds the hook there; the change is localized:
   extend `InstanceData` stride 64→80 bytes, add the field, mix into
   fragment color in `mesh_modern.frag`. ~30 min when the time comes.
+- `src/AcDream.App/Rendering/TerrainModernRenderer.cs` — terrain dispatcher
+  on N.5's modern primitives. Mirrors WB's `TerrainRenderManager` pattern
+  (single global VBO/EBO + slot allocator + `glMultiDrawElementsIndirect`)
+  but driven by acdream's `LandblockMesh.Build` so retail's `FSplitNESW`
+  formula is preserved (issue #51 resolved). Atlas handles bound via the
+  uvec2 + `sampler2DArray(handle)` constructor pattern (NOT the direct
+  `uniform sampler2DArray` + `glProgramUniformHandleARB` form, which
+  GL_INVALID_OPERATIONs on at least one driver).
 
 **Execution phases:** R1→R8 in the architecture doc. Each phase has clear
 goals, test criteria, and builds on the previous. Don't skip phases.
@@ -504,13 +512,33 @@ acdream's plan lives in two files committed to the repo:
 
 **Currently in flight: Phase N.6 — Perf polish.**
 Roadmap entry at [`docs/plans/2026-04-11-roadmap.md`](docs/plans/2026-04-11-roadmap.md).
-Builds on N.5. Legacy renderers (`InstancedMeshRenderer`, `StaticMeshRenderer`,
-`WbFoundationFlag`) were retired in the N.5 ship amendment — N.6 scope is
-perf-only: WB atlas adoption, persistent-mapped buffers, GPU-side culling,
-GL_TIME_ELAPSED query double-buffering, direct N.4 vs N.5 perf measurement,
-legacy `Texture2D`/`sampler2D` TextureCache path retirement (Sky/Terrain/Debug).
+Builds on N.5 + N.5b. Legacy renderers (`InstancedMeshRenderer`,
+`StaticMeshRenderer`, `WbFoundationFlag`) were retired in the N.5 ship
+amendment, and the terrain legacy renderer (`TerrainChunkRenderer` +
+`TerrainRenderer` + legacy `terrain.vert/.frag`) was retired in N.5b.
+N.6 scope is perf-only: WB atlas adoption, persistent-mapped buffers
+(strong candidate after N.5b's per-frame DEIC `BufferSubData`),
+GPU-side culling via compute pre-pass, GL_TIME_ELAPSED query
+double-buffering, direct higher-radius perf comparison once A.5 lands,
+legacy `Texture2D`/`sampler2D` TextureCache path retirement (Sky / Debug
+remain on the legacy path now that Terrain has migrated).
 Plan + spec written when work begins.
 
+**Phase N.5b (Terrain on Modern Rendering Path) shipped 2026-05-09.**
+`TerrainModernRenderer` mirrors WB's `TerrainRenderManager` pattern
+(single global VBO/EBO + slot allocator + bindless atlas +
+`glMultiDrawElementsIndirect`) but consumes `LandblockMesh.Build` so
+retail's `FSplitNESW` formula is preserved (Path C; closes ISSUE #51).
+Path A (substitute WB's `CalculateSplitDirection`) killed by 49.98%
+divergence vs retail in
+[`tests/AcDream.Core.Tests/Terrain/SplitFormulaDivergenceTest.cs`](tests/AcDream.Core.Tests/Terrain/SplitFormulaDivergenceTest.cs).
+At radius=5 in Holtburg modern is ~4× SLOWER on CPU than the legacy
+chunked path was; architectural wins manifest at higher radius. Honest
+perf baseline at
+[`docs/plans/2026-05-09-phase-n5b-perf-baseline.md`](docs/plans/2026-05-09-phase-n5b-perf-baseline.md).
+Plan archived at
+[`docs/superpowers/plans/2026-05-09-phase-n5b-terrain-modern.md`](docs/superpowers/plans/2026-05-09-phase-n5b-terrain-modern.md).
+
 **Phase N.5 (Modern Rendering Path) shipped + amended 2026-05-08.** `WbDrawDispatcher`
 on bindless textures + `glMultiDrawElementsIndirect`. CPU dispatcher 1.23ms/frame
 at Holtburg (~810 fps). **Ship amendment:** `InstancedMeshRenderer`,