phase(N.0): wire up WorldBuilder fork as submodule + project refs

Phase N.0 setup for the WorldBuilder migration. Replaces the local
read-only clone of Chorizite/WorldBuilder at references/WorldBuilder/
with a git submodule pointing at our fork
(github.com/eriknihlen/WorldBuilder.git, branch acdream).

Changes:
- .gitignore: exempt references/WorldBuilder from the references/
  ignore rule so the submodule can be tracked.
- .gitmodules (new): submodule entry tracking acdream branch on fork.
- src/AcDream.Core/AcDream.Core.csproj: add ProjectReference to
  WorldBuilder.Shared and Chorizite.OpenGLSDLBackend so we can call
  TerrainUtils, SceneryHelpers, etc. from our Core code.

Build green, all 93 scenery/terrain tests pass. The 8 pre-existing
DispatcherToMovement test failures are unrelated and exist on main.

Notes for users picking up this branch on main:
- After merge, the existing local clone at references/WorldBuilder
  may need to be removed before `git submodule update --init` will
  populate the submodule.
- Working on the fork happens via `cd references/WorldBuilder && git
  checkout acdream && <changes> && git push`. To pull upstream
  Chorizite/WorldBuilder fixes: `git remote add upstream
  https://github.com/Chorizite/WorldBuilder.git && git fetch upstream
  && git merge upstream/master`.

Next: Phase N.1 — replace SceneryGenerator algorithm calls with
WB's SceneryHelpers + TerrainUtils.

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

.gitignore

@@ -18,7 +18,11 @@ packages/
 Thumbs.db
 
 # Reference repos and retail client (large, not our code, separate licenses)
-references/
+# WorldBuilder is exempt — it's a load-bearing dependency tracked as a git
+# submodule pointing at our fork (Phase N, see docs/architecture/worldbuilder-inventory.md).
+references/*
+!references/WorldBuilder
+!references/WorldBuilder/
 
 # Claude Code session state
 .claude/