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>

docs/ISSUES.md

@@ -46,64 +46,6 @@ Copy this block when adding a new issue:
 
 # Active issues
 
-## #51 — WB's terrain-split formula diverges from retail's `FSplitNESW`
-
-**Status:** OPEN
-**Severity:** MEDIUM (blocks isolated N.2; affects sequencing of N-phase migration)
-**Filed:** 2026-05-08
-**Component:** terrain math / Phase N (WorldBuilder rendering migration)
-
-**Description:** WB's `TerrainUtils.CalculateSplitDirection`
-([references/WorldBuilder/WorldBuilder.Shared/Modules/Landscape/Lib/TerrainUtils.cs:44](references/WorldBuilder/WorldBuilder.Shared/Modules/Landscape/Lib/TerrainUtils.cs:44))
-uses a different math expression from retail's `FSplitNESW`
-(documented in CLAUDE.md as **the** real AC terrain split formula,
-constants `0x0CCAC033` / `0x421BE3BD` / `0x6C1AC587` / `0x519B8F25`).
-Ours is a degree-2 polynomial in (x,y); WB's is linear in (x,y).
-They cannot be algebraically equivalent and disagree on a meaningful
-fraction of cells.
-
-**Concrete impact:** On any cell where the formulas pick different
-diagonals, the same world position (X, Y) maps to different terrain
-heights — up to ~2m for a sloped cell with one elevated corner. If a
-caller mixes "WB-formula path" and "AC2D-formula path" for the same
-cell, the player physics floats above or sinks below the visible
-ground. This is the bug class fixed in
-[src/AcDream.Core/Physics/TerrainSurface.cs:113-120](src/AcDream.Core/Physics/TerrainSurface.cs:113)
-(diagonal-direction inversion).
-
-**Files implicated:**
-- `src/AcDream.Core/Physics/TerrainSurface.cs` — uses AC2D formula via
-  `IsSplitSWtoNE`
-- `src/AcDream.Core/World/TerrainBlending.cs` — visual mesh, also AC2D
-- `references/WorldBuilder/WorldBuilder.Shared/Modules/Landscape/Lib/TerrainUtils.cs:44`
-  — WB's diverging formula
-- `references/WorldBuilder/Chorizite.OpenGLSDLBackend/Lib/TerrainGeometryGenerator.cs`
-  — WB's render mesh (presumably also uses WB's formula in lockstep)
-
-**Sequencing implication:** Phase N.2 (terrain math helpers
-substitution) cannot be shipped in isolation — it must land alongside
-visual terrain renderer migration (originally N.5, now moved to N.7
-scope), at which point both physics and visual mesh switch to WB's
-formula together. N.5 shipped entity rendering only; terrain remains
-on acdream's own pipeline through N.7.
-
-**Research needed (when N.7 picks this up):**
-1. Quantify divergence: run WB's `CalculateSplitDirection` and our
-   `IsSplitSWtoNE` across all (lbX, lbY, cellX, cellY) tuples for a
-   representative landblock set; record disagreement rate.
-2. Confirm WB's `TerrainGeometryGenerator` uses WB's formula in its
-   render mesh — if so, switching everything to WB's formula keeps
-   visual + physics synced. (Highly likely.)
-3. Decide whether ANY retail-conformance test (e.g., physics matching
-   server-authoritative Z within tolerance) is invalidated by the
-   formula change.
-
-**Acceptance:** Resolved when N.7 lands and both physics + visual
-terrain use WB's split formula, OR when we decide to keep the AC2D
-formula and patch WB's renderer in our fork.
-
----
-
 ## #50 — Road-edge tree at 0xA9B1 visible in acdream but not retail
 
 **Status:** OPEN
@@ -1758,6 +1700,57 @@ Unverified. The likely culprits, ranked by suspected probability:
 
 # Recently closed
 
+## #51 — [DONE 2026-05-09 · da56063 + N.5b SHIP] WB's terrain-split formula diverges from retail's `FSplitNESW`
+
+**Closed:** 2026-05-09
+**Commit:** `da56063` (black-terrain fix; landed within Phase N.5b — see
+`docs/superpowers/plans/2026-05-09-phase-n5b-terrain-modern.md` for the
+ship commit chain)
+**Component:** terrain math / Phase N.5b
+
+**Resolution: Path C.** Phase N.5b lifted terrain rendering onto the
+modern path (bindless atlas + `glMultiDrawElementsIndirect`) WITHOUT
+adopting WB's `TerrainUtils.CalculateSplitDirection`. The pre-implementation
+divergence test (`tests/AcDream.Core.Tests/Terrain/SplitFormulaDivergenceTest.cs`)
+confirmed the two formulas disagree on **49.98%** of sweep cells —
+fundamentally incompatible with our shared physics + visual mesh, which
+both rely on retail's `FSplitNESW` (constants `0x0CCAC033` / `0x421BE3BD` /
+`0x6C1AC587` / `0x519B8F25`).
+
+Path C: keep retail's `FSplitNESW` formula via `LandblockMesh.Build` →
+`TerrainBlending.CalculateSplitDirection`; mirror WB's `TerrainRenderManager`
+architectural pattern (single global VBO/EBO + slot allocator + bindless
+atlas + multi-draw indirect) but feed it acdream's mesh. Modern dispatcher
+(`TerrainModernRenderer`) replaces `TerrainChunkRenderer` (deleted in T9
+along with `TerrainRenderer` + `terrain.vert/.frag`).
+
+Path A (substitute WB's formula) was killed by the divergence test.
+Path B (fork-patch WB's renderer to use retail's formula) was rejected
+for permanent maintenance burden. Path C ships the architectural
+pattern while preserving retail-formula compliance.
+
+Visual mesh and physics both still consume retail's `FSplitNESW`; they
+remain in lockstep, no triangle-Z hover. The N.6 / N.7 sequencing
+implication this issue carried (substitute physics math only when the
+visual mesh migrates) is moot — neither side ever switches to WB's
+formula.
+
+**Files added:**
+- `src/AcDream.App/Rendering/TerrainModernRenderer.cs`
+- `src/AcDream.Core/Terrain/TerrainSlotAllocator.cs`
+- `src/AcDream.App/Rendering/Shaders/terrain_modern.vert`
+- `src/AcDream.App/Rendering/Shaders/terrain_modern.frag`
+- `tests/AcDream.Core.Tests/Terrain/SplitFormulaDivergenceTest.cs` (the
+  test that killed Path A)
+
+**Files deleted (T9):**
+- `src/AcDream.App/Rendering/TerrainChunkRenderer.cs`
+- `src/AcDream.App/Rendering/TerrainRenderer.cs`
+- `src/AcDream.App/Rendering/Shaders/terrain.vert`
+- `src/AcDream.App/Rendering/Shaders/terrain.frag`
+
+---
+
 ## #43 — [DONE 2026-05-05 · 9e4772a] Slope staircase on observed player remotes (anim-only fallback ignored slope)
 
 **Closed:** 2026-05-05