docs: roadmap N.0 shipped + realistic N.2-N.9 estimates + N.3 handoff

Roadmap updates after Phase N.1 ship:
- Marks N.0 (submodule + project refs setup) as ✓ SHIPPED with the
  c8782c9 commit reference
- Updates N.2-N.9 effort estimates with realistic post-N.1 numbers
  (originals were 1-2 days / 1 week / 2 weeks; realistic numbers
  factor in conformance-test discovery, ACME-vs-Chorizite delta
  hunts, and the visual-verification-then-revert cycle that ate
  most of N.1's calendar time)
- Adds a "Lessons from N.1" subsection so future N phases benefit
  from the rotation-bug-conformance-test pattern, the ACME divergence
  insight, and the "whackamole = stop" rule
- Updates total calendar estimate to 3-4 months / 10-12 engineering
  weeks for N.2-N.9 (was 2-3 months / 6-8 weeks)

New handoff doc at docs/research/2026-05-08-phase-n3-handoff.md
captures everything a fresh agent picking up N.3 (texture decoding)
needs: phase context, what to read first, suggested task decomposition,
watchouts (especially the ACME-divergence and conformance-test
lessons), and where to start.

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

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

@@ -530,9 +530,39 @@ 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.
 
+**Lessons from N.1 (apply to N.2-N.10):**
+
+1. **Per-helper conformance tests work.** The N.1 conformance test caught a
+   ~180° rotation bug in our retail port that had been silently wrong
+   forever. Write the conformance test BEFORE the substitution in each
+   sub-phase.
+
+2. **ACME ≠ Chorizite/WorldBuilder.** ACME is a downstream fork of WB with
+   additional retail-faithful filters that upstream WB (our submodule)
+   doesn't have. When a visual discrepancy appears, check ACME's source
+   (`references/WorldBuilder-ACME-Edition/`) for delta filters BEFORE
+   investigating retail decomp directly. ACME's deltas tend to come as
+   coherent units — porting one filter without its companions can
+   over-suppress.
+
+3. **"Whackamole" is the warning sign.** If a phase generates 3+ visual
+   regressions on default-on, stop, accept the cosmetic deltas as
+   ISSUES.md entries, ship the migration. Bugs we leave behind are
+   debuggable; bugs we never ship are forgotten.
+
+4. **Subagent-driven execution holds up at this scope.** Fresh subagent
+   per task with the full task text inline keeps quality high without
+   polluting the controller's context. Each task should be self-contained
+   enough that a subagent without session history can complete it.
+
 **Sub-phases (strangler-fig with feature flags):**
 
-- **N.0 — Setup.** Submodule + project references + build green. ~1-2 hrs.
+- **✓ SHIPPED — N.0 — Setup.** Shipped 2026-05-08 (commit `c8782c9`).
+  WorldBuilder fork at `github.com/eriknihlen/WorldBuilder.git` registered
+  as git submodule at `references/WorldBuilder/` tracking the `acdream`
+  branch. `AcDream.Core.csproj` references `WorldBuilder.Shared` +
+  `Chorizite.OpenGLSDLBackend`. Build green, all 28 scenery/terrain tests
+  passing.
 - **✓ SHIPPED — N.1 — Scenery algorithm calls.** Shipped 2026-05-08.
   Replaced `IsOnRoad` / `DisplaceObject` / slope-normal calc / rotation /
   scale inside `SceneryGenerator.Generate()` with calls to WB's
@@ -545,31 +575,50 @@ for our deletions/additions; merge upstream `master` periodically.
   (road-edge tree at landblock 0xA9B1).
 - **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.
+  / `GetNormal` internally. ~1-2 days. Smallest remaining N phase, low
+  risk after N.1's conformance proof on GetNormal.
+- **N.3 — Texture decoding.** Replace our `TextureCache` decode
+  pipeline (`src/AcDream.App/Rendering/TextureCache.cs`) with WB's
+  `TextureHelpers` (INDEX16, P8, BGRA, DXT, alpha). Touches every
+  texture path. **Realistic estimate: 3-5 days** (was 2-3) — the GL
+  upload path needs adapting and we'll need conformance tests per
+  texture format. Handoff doc:
+  `docs/research/2026-05-08-phase-n3-handoff.md`.
 - **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.
+  ours — ACME is the secondary oracle. **Realistic estimate: 1.5-2
+  weeks** (was 1) — character appearance edge cases like N.1's
+  rotation bug will surface.
 - **N.5 — Terrain rendering.** Replace `TerrainChunkRenderer` +
   `TerrainAtlas` + `TerrainBlending` with WB's `TerrainRenderManager` +
-  `LandSurfaceManager` + `TerrainGeometryGenerator`. ~2 weeks.
+  `LandSurfaceManager` + `TerrainGeometryGenerator`. **Realistic
+  estimate: 3-4 weeks** (was 2) — largest single phase, GPU-buffer
+  ownership shifts, integration with our streaming loader is
+  non-trivial.
 - **N.6 — Static objects rendering.** Replace `StaticMeshRenderer` +
   `InstancedMeshRenderer` with WB's `StaticObjectRenderManager`.
-  ~2 weeks.
+  **Realistic estimate: 2-3 weeks** (was 2) — interacts with N.4
+  output.
 - **N.7 — EnvCells / dungeons.** Replace EnvCell rendering with WB's
-  `EnvCellRenderManager` + `PortalRenderManager`. ~2 weeks.
+  `EnvCellRenderManager` + `PortalRenderManager`. **Realistic
+  estimate: 2-3 weeks** (was 2).
 - **N.8 — Sky + particles.** Replace sky rendering + particle pipeline
   (#36 / C.1 work) with WB's `SkyboxRenderManager` +
-  `ParticleEmitterRenderer`. ~1 week.
+  `ParticleEmitterRenderer`. **Realistic estimate: 1.5-2 weeks**
+  (was 1) — visual continuity matters; we just shipped C.1 and that
+  work flows through here.
 - **N.9 — Visibility / culling.** Replace `CellVisibility` +
-  `FrustumCuller` with WB's `VisibilityManager`. ~3-5 days.
+  `FrustumCuller` with WB's `VisibilityManager`. **Realistic
+  estimate: 1 week** (was 3-5 days) — affects perf and what gets
+  drawn.
 - **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.
+**Estimated calendar:** **3-4 months / 10-12 engineering weeks for
+N.2-N.9 (skipping N.10).** (Was 2-3 months / 6-8 weeks — revised
+upward after N.1 landed; realistic per-phase numbers above.)
 
 **Each sub-phase:**
 - Ships behind `ACDREAM_USE_WB_<NAME>=1` flag.