spec(rendering): Phase N WorldBuilder migration design + N.1 scenery

Adds two design docs and a roadmap entry for the strategic shift
from "port retail rendering algorithms ourselves" to "depend on a
fork of Chorizite/WorldBuilder for rendering + dat-handling."

- docs/superpowers/specs/2026-05-08-phase-n-worldbuilder-migration-design.md
  — parent design: integration model (fork + submodule), 10 sub-phases
  (N.0 setup through N.10 GL consolidation), strangler-fig phasing
  with per-phase feature flags, retail-decomp boundary clarified for
  what WB does NOT cover (network, physics, animation, motion, UI,
  plugin, audio, chat).

- docs/superpowers/specs/2026-05-08-phase-n1-scenery-via-wb-helpers-design.md
  — N.1 detailed design: replace IsOnRoad / DisplaceObject /
  slope-normal calc / rotation / scale inside SceneryGenerator.Generate()
  with calls to WB's SceneryHelpers + TerrainUtils. Keep data flow,
  ScenerySpawn shape, and renderer integration. Add small LandBlock →
  TerrainEntry[] adapter. Feature flag ACDREAM_USE_WB_SCENERY=1.

- docs/plans/2026-04-11-roadmap.md — Phase N entry added between
  Phase M and Phase J. Lists all 10 sub-phases with effort estimates.

Fork already created at https://github.com/eriknihlen/WorldBuilder.
N.0 setup (replace references/WorldBuilder/ snapshot with submodule,
add project references, build green) is the next implementation step.

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

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

@@ -498,6 +498,93 @@ before porting.
 
 ---
 
+### Phase N — WorldBuilder Rendering Migration
+
+**Goal:** Stop re-porting AC-specific rendering / dat-handling
+algorithms. Depend on a fork of `Chorizite/WorldBuilder` (MIT) for
+terrain, scenery, static objects, EnvCells, portals, sky, particles,
+texture decoding, mesh extraction, and visibility. Acdream keeps its
+own network, physics, animation, motion, UI, plugin, audio, chat
+layers (those aren't in WB).
+
+**Why now (2026-05-08):** the scenery edge-vertex bug at landblock
+`0xA9B1` was the third subtle porting bug in a quarter (after the
+triangle-Z bug and the hover-over-terrain bug). Even when our code
+looked byte-identical to WB's, our output diverged. WB renders the
+world correctly; the cost of "we re-port retail algorithms" is now
+higher than "we depend on WB's tested port."
+
+**Design + inventory:**
+
+- `docs/architecture/worldbuilder-inventory.md` — full taxonomy of
+  what WB has and what we keep porting ourselves.
+- `docs/superpowers/specs/2026-05-08-phase-n-worldbuilder-migration-design.md` —
+  parent design doc.
+- `docs/superpowers/specs/2026-05-08-phase-n1-scenery-via-wb-helpers-design.md` —
+  N.1 detailed design.
+
+**Integration model:** fork at
+`https://github.com/eriknihlen/WorldBuilder` (already created), git
+submodule replacing `references/WorldBuilder/` snapshot, project
+references in our solution. Long-lived `acdream` branch in the fork
+for our deletions/additions; merge upstream `master` periodically.
+
+**Sub-phases (strangler-fig with feature flags):**
+
+- **N.0 — Setup.** Submodule + project references + build green. ~1-2 hrs.
+- **N.1 — Scenery algorithm calls.** Replace `IsOnRoad` /
+  `DisplaceObject` / slope-normal calc / rotation / scale inside
+  `SceneryGenerator.Generate()` with calls to WB's `SceneryHelpers` +
+  `TerrainUtils`. Tiny adapter `LandBlock → TerrainEntry[]`. Keeps our
+  data flow + `ScenerySpawn` shape. Feature flag
+  `ACDREAM_USE_WB_SCENERY=1`. ~1-2 days.
+- **N.2 — Terrain math helpers.** Refactor `TerrainSurface.SampleZ` /
+  `SampleNormal` / `SampleSurface` to call WB's `TerrainUtils.GetHeight`
+  / `GetNormal` internally. ~1-2 days.
+- **N.3 — Texture decoding.** Replace `TextureCache` decode pipeline
+  with WB's `TextureHelpers`. ~2-3 days.
+- **N.4 — Object meshing.** Replace `SetupMesh.cs` + `GfxObjMesh.cs`
+  with calls to WB's `ObjectMeshManager`. Character-appearance
+  behaviors (CreaturePalette / GfxObjRemapping / HiddenParts) remain
+  ours — ACME is the secondary oracle. ~1 week.
+- **N.5 — Terrain rendering.** Replace `TerrainChunkRenderer` +
+  `TerrainAtlas` + `TerrainBlending` with WB's `TerrainRenderManager` +
+  `LandSurfaceManager` + `TerrainGeometryGenerator`. ~2 weeks.
+- **N.6 — Static objects rendering.** Replace `StaticMeshRenderer` +
+  `InstancedMeshRenderer` with WB's `StaticObjectRenderManager`.
+  ~2 weeks.
+- **N.7 — EnvCells / dungeons.** Replace EnvCell rendering with WB's
+  `EnvCellRenderManager` + `PortalRenderManager`. ~2 weeks.
+- **N.8 — Sky + particles.** Replace sky rendering + particle pipeline
+  (#36 / C.1 work) with WB's `SkyboxRenderManager` +
+  `ParticleEmitterRenderer`. ~1 week.
+- **N.9 — Visibility / culling.** Replace `CellVisibility` +
+  `FrustumCuller` with WB's `VisibilityManager`. ~3-5 days.
+- **N.10 — GL infrastructure consolidation (optional).** Replace our
+  `Shader` / `TextureCache` / `SamplerCache` plumbing with WB's
+  `ManagedGL*` wrappers + `OpenGLGraphicsDevice`. ~1 week.
+
+**Estimated calendar:** 2-3 months. Engineering effort: 6-8 weeks.
+
+**Each sub-phase:**
+- Ships behind `ACDREAM_USE_WB_<NAME>=1` flag.
+- Has its own conformance test (side-by-side against existing path).
+- Visual verification before flag becomes default-on.
+- Old code deleted after default-on lands cleanly.
+
+**N.2-N.10 detailed specs are NOT yet written** — each gets its own
+brainstorm + spec when we reach it.
+
+**Acceptance:**
+- All 10 sub-phases shipped, feature flags removed, old rendering code
+  paths deleted.
+- Visual verification at Holtburg + Foundry statue + a representative
+  dungeon shows no regression vs Phase C.1.
+- WB upstream merges into our `acdream` branch are clean (or have
+  documented conflict-resolution patterns).
+
+---
+
 ### Phase J — Long-tail (deferred / low-priority)
 
 Not detailed here; each gets its own brainstorm when it becomes relevant.