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>
