feat: sprite-mode render with z_order sort and color-rect fallback

Walks bones in z-sorted order (rig.bones_z_sorted, lowest first =
back). For each bone:
- If texture_handle present: draw via engine.render.draw_sprite_transform
  using bone.scale and bone.anchor (negative scale mirrors the sprite,
  anchor is the in-texture pivot).
- Else: fall back to colored rotated 16x4 rect via draw_rect_rotated
  (Phase 1 behavior preserved).

bone_state[bid].world (cached by M.update's depth-first pipeline) is
the source of (x, y, angle).

init.lua

@@ -542,27 +542,32 @@ update_bone_recursive = function(p, b, dt)
 end
 
 -- ====================================================================
--- Rendering (engine.render.* -- only callable in render-phase)
+-- Rendering (engine.render.*; only callable in render-phase).
+-- Walks bones in z-sorted order (back-to-front). For each bone:
+--   - If bone has texture_handle: draw via engine.render.draw_sprite_transform
+--   - Else: fall back to engine.render.draw_rect_rotated with bone.color
 -- ====================================================================
 function M.render(handle)
     local p = all_puppets[handle]
     if p == nil then return end
 
-    -- Draw each bone as a rotated rect centered at the bone's world-pos.
-    -- v0.1 simplification: bone's world-pos = puppet origin + bone.rest offset.
-    -- Bone "length" is approximated by a small constant; later versions will
-    -- derive from rig data.
-    local BONE_LENGTH = 16
-    local BONE_THICKNESS = 4
-
-    for _, b_src in ipairs(p.rig.bones) do
-        local b = p.rig.bones_by_id[b_src.id]
-        local x = p.x + b.rest.x
-        local y = p.y + b.rest.y
-        local angle = p.bone_state[b.id].angle
-        local c = b.color
-        local color = engine.render.rgb(c[1], c[2], c[3])
-        engine.render.draw_rect_rotated(x, y, BONE_LENGTH, BONE_THICKNESS, angle, color)
+    for _, b in ipairs(p.rig.bones_z_sorted) do
+        local ws = p.bone_state[b.id].world
+        if b.texture_handle then
+            local sx, sy = b.scale[1], b.scale[2]
+            local ax, ay = b.anchor and b.anchor[1] or 0,
+                           b.anchor and b.anchor[2] or 0
+            engine.render.draw_sprite_transform(
+                b.texture_handle,
+                ws.x, ws.y, ws.angle,
+                sx, sy, ax, ay,
+                0xFFFFFFFF
+            )
+        else
+            -- Phase-1 fallback: colored rotated rect (16x4 px).
+            local color = engine.render.rgb(b.color[1], b.color[2], b.color[3])
+            engine.render.draw_rect_rotated(ws.x, ws.y, 16, 4, ws.angle, color)
+        end
     end
 end