Add auto-tiling walls, multi-floor buildings, CA cave gen, and enhanced dungeons

[phaax]

• Auto-tiling wall system (autotile.ts): walls now render as context-aware box-drawing
  characters (╭╮╰╯ rounded corners, ─│ runs, ├┤┬┴┼ junctions) based on neighbours.
  Runs at render time only — collision is unchanged.

• Multi-floor buildings (towngen.ts): inns and guilds get 2 upper floors, temples get 1.
  Each upper floor is a subdivided layout with beds/furniture and return stairs (cyan >).
  Service NPCs (innkeeper, shopkeeper, healer, etc.) are ground-floor only. Generic and
  quest NPCs may appear on upper floors (30%/20% chance). Upper floors have a 20% chance
  of containing a chest.

• Basements (towngen.ts): taverns (15%) and inns (10%) may have a basement (floor -1).
  Basements have 8% chance of monsters (with guaranteed chest), 5% chance of a quest NPC,
  and 15% independent chest chance. Encounter rate in basements is 20% of dungeon rate.

• Floor transition (location.ts + game.ts): STAIRS_FLOOR_UP (cyan <) ascends,
  STAIRS_FLOOR_DOWN (cyan >) descends. game.currentFloor / currentFloorData /
  currentFloorEntry track the active floor. All tile/NPC/chest lookups route through
  the active floor data. Player.currentFloor persists through save/load.

• Floor dimming (location.ts): when currentFloor ≠ 0 (upper floors or basement), tiles
  outside the current building's bounding box render with fg = DARK_GRAY, making the
  outdoor environment appear dim.

• Cellular automata cave generation (dungeogen.ts): caves now use a CA algorithm
  (45% random fill → 4 smooth passes → keep largest connected region) for organic,
  non-rectangular cave shapes instead of BSP rectangular rooms.

• Dungeon room variety: 10% chance of cross-shaped rooms, 10% chance of wide corridor
  halls, in addition to standard rectangular rooms.

• New decorations: caves get CAVE_WATER pools (≈), STALACTITE drips (i), MUSHROOM (°),
  and CRYSTAL (*) tiles. Dungeons get RUBBLE (%) in corners and CRYSTAL on walls.
  Ruins now use RUBBLE instead of floor holes for a better appearance.

• 7 new LOC_TILE values (24-30), updated isWalkableTile, new FloorData/BuildingFloorEntry
  types, DUNGEON_TILES and TOWN_TILES entries for all new tile types.

All 59 existing unit tests pass. Build succeeds cleanly.

https://claude.ai/code/session_01ER9P4Z8PP6r2cKjfUhoUgy