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>

demo/scenes/demo_blobber.tscn

@@ -1,7 +1,7 @@
 [gd_scene format=3 uid="uid://c8blbrbrbr001"]
 
 [ext_resource type="Script" uid="uid://b2r6hnyvt7ef7" path="res://scripts/dungeon_builder.gd" id="1_builder"]
-[ext_resource type="Script" uid="uid://d0srrm35g1m0t" path="res://scripts/fly_camera.gd" id="2_flycam"]
+[ext_resource type="Script" path="res://scripts/blobber_party.gd" id="2_party"]
 
 [sub_resource type="Environment" id="Environment_1"]
 background_mode = 1
@@ -15,8 +15,9 @@ ambient_light_energy = 0.5
 [node name="Dungeon" type="Node3D" parent="." unique_id=2089249369]
 script = ExtResource("1_builder")
 seed_value = 43
-num_levels = 8
+num_levels = 3
 depth = 20
+party_path = NodePath("../Party")
 
 [node name="DirectionalLight3D" type="DirectionalLight3D" parent="." unique_id=1913401313]
 transform = Transform3D(0.86602527, -0.35310844, 0.35399818, 0, 0.70799595, 0.70621645, -0.5000003, -0.6116013, 0.6131424, 80, 40, 80)
@@ -25,9 +26,10 @@ shadow_enabled = true
 [node name="WorldEnvironment" type="WorldEnvironment" parent="." unique_id=1337946831]
 environment = SubResource("Environment_1")
 
-[node name="FlyCamera" type="Camera3D" parent="." unique_id=987300286]
-transform = Transform3D(0.9, 0, 0, 0, 0.7, 0.7, 0, -0.7, 0.7, 120, 40, 80)
-fov = 65.0
+[node name="Party" type="Node3D" parent="." unique_id=987300286]
+script = ExtResource("2_party")
+
+[node name="Camera3D" type="Camera3D" parent="Party"]
+fov = 75.0
 near = 0.1
 far = 500.0
-script = ExtResource("2_flycam")