feat: discrete party controller + aligned stair pairs (PR 2)

Adds the Wizardry/M&M core loop: you walk cell-by-cell with 90° turns
and descend stairs between levels that actually line up.

C side:
- pipeline.c: after per-level 2D generation, a link_stairs() pass
  replaces the randomly-placed down/up stairs with aligned pairs
  (room cells preferred). Bottom level loses its down-stair; top
  level keeps the up-stair as the entry point.
- dungeon_to_dict.cpp: expose sizeof(cell3d_t) as "cell_stride" so
  GDScript can index raw cell bytes without hardcoding layout.

Godot side:
- scripts/blobber_party.gd: reads cell3d_t bytes directly for wall
  queries, tweens position/rotation on step/turn, swaps level when
  stair cell is activated.
- scripts/dungeon_builder.gd: now hands the generated Dictionary to
  a party node via `party_path` and groups mesh instances under a
  "Meshes" child for clean regeneration.
- scenes/demo_blobber.tscn: FlyCamera replaced with a Party node
  (script-driven) holding a child Camera3D. num_levels=3 by default.

Still deferred to later PRs: the full port/retirement of src/gen/,
and a standalone plan.c/h module (linkage is currently inlined in
pipeline.c with just StairPair-equivalent data).

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

godot/src/dungeon_to_dict.cpp

@@ -88,6 +88,7 @@ Dictionary godot::dungeon_to_dictionary(const dungeon_t *d, const mesh_build_t *
     out["depth"]      = depth;
     out["num_levels"] = num_levels;
     out["cell_size"]  = mb->cell_size;
+    out["cell_stride"] = (int)sizeof(cell3d_t);
     out["dimensions"] = Vector3i(BL_DCOLS, num_levels, BL_DROWS);
 
     // Raw voxel cell data for future mutation. Order: z-major (level,y,x).