chore: phase 0 — skeleton + dat asset inventory

Brand-new solution targeting .NET 10, using Chorizite.DatReaderWriter 2.1.4
to walk a retail AC dat directory and print how many of each asset type live
in client_portal / client_cell_1 / client_highres / client_local_English.

Opens the four dats in ~16 ms and counts 887,381 indexed assets across 40+
tracked DBObj types. Cell-database terrain (LandBlock, LandBlockInfo, EnvCell)
uses mask-based IDs that DatReaderWriter 2.1.4's GetAllIdsOfType<T> does not
support; worked around with a manual b-tree walk in CountCellByLow16.

Sanity check: LandBlock count is 65,025 = 255 x 255, exactly the AC world grid.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,24 @@
+# Build output
+bin/
+obj/
+out/
+
+# Rider / VS
+.idea/
+.vs/
+*.user
+*.suo
+
+# NuGet
+*.nupkg
+packages/
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Reference repos and retail client (large, not our code, separate licenses)
+references/
+
+# Claude Code session state
+.claude/