feat: sprite-mode test-chamber with 13-bone puppet + sprite-tilemap

13-bone humanoid rig (12 visible bones + 1 root) hand-ported from
substrate reference, mapped to PROTOTYPE asset-pack via asset_aliases
indirection (asset_pack: player). Bones use texture/z_order/scale/
anchor fields from the rig-format extension. Tracks split: lower for
legs + feet (walk animation), upper for root/body/head/shoulders/arms
(idle animation).

Animations:
- idle.anim.json: 2.0s loop on upper track, subtle body+head sway
- walk.anim.json: 0.6s loop on lower track, legs+feet cycle

Sprite-tilemap: 4-tile palette (floor_001-003 walkable, floor_004
as wall). Map content: 20x20 with wall-border and interior wall
blocks. tile_rotations parallel array exercises 90 and 180-degree
rotations on a handful of cells for visual rotation-pipeline
verification.

init.lua loads textures via asset_aliases lookup:
- maps.load_textures(aliases) resolves tilemap-tile atlas-ids
- puppet.load_rig(path, aliases) loads + resolves bone textures
- furniture textures preloaded at init (bed, bench, backpack) to
  avoid first-frame load stutter; rendered via engine.render.
  draw_sprite_transform in the draw hook

Camera follows puppet via per-frame camera.set_target. Headtrack:
head bone (look_at: true) rotates to face mouse world-position.

New SPOREL_CI=1 traces from render and init hooks:
- vagrant: render_frame_ok (per frame from render)
- vagrant: sprite_mode=on (once after first sprite-draw)
- vagrant: assets_loaded=14 (once after init: 7 puppet + 4 tile + 3 furniture)

animations/walk.anim.json

@@ -4,8 +4,11 @@
     "duration": 0.6,
     "loop": true,
     "keyframes": [
-        { "t": 0.0, "leg_l": { "angle": -15 }, "leg_r": { "angle":  15 } },
-        { "t": 0.3, "leg_l": { "angle":  15 }, "leg_r": { "angle": -15 } },
-        { "t": 0.6, "leg_l": { "angle": -15 }, "leg_r": { "angle":  15 } }
+        { "t": 0.0, "leg_l":  {"angle": -20}, "leg_r":  {"angle":  20},
+                    "foot_l": {"angle":  10}, "foot_r": {"angle": -10} },
+        { "t": 0.3, "leg_l":  {"angle":  20}, "leg_r":  {"angle": -20},
+                    "foot_l": {"angle": -10}, "foot_r": {"angle":  10} },
+        { "t": 0.6, "leg_l":  {"angle": -20}, "leg_r":  {"angle":  20},
+                    "foot_l": {"angle":  10}, "foot_r": {"angle": -10} }
     ]
 }