feat: extend rig-format with texture/z_order/scale/anchor fields

Additive extension to build_rig output, preserving backward-compat
with existing rigs that lack the new fields:

- bone.texture: optional atlas-id (string) for sprite-mode rendering;
  resolved to a texture-handle by puppet.load_textures(). When absent
  the bone falls back to colored-rect render via draw_rect_rotated.
- bone.z_order: render-sort key (lower = back, higher = front);
  default 0.
- bone.scale: [sx, sy] pair; default [1, 1]; negative components
  mirror the sprite.
- bone.anchor: optional [x, y] override of the atlas-supplied anchor.
- rig.asset_pack: alias-key into the consumer module's asset_aliases
  manifest field; lets puppet.load_textures resolve atlas-ids to
  texture-handles via the asset-lib.
- rig.bones_z_sorted: array of bones pre-sorted by z_order; render
  walks this once per frame.

Version bumped to 0.2.0. Subsequent commits add multi-level parent-
chain transform, sprite-mode render, and load_textures.

init.lua

@@ -1,4 +1,4 @@
--- lib-core.puppet v0.1.0
+-- lib-core.puppet v0.2.0
 -- Skeletal animation: skeleton + bones + tracks + rest + keyframe
 -- + procedural + look-at constraint. No footplant in v0.1.
 
@@ -67,6 +67,11 @@ function M.build_rig(rig_table)
             },
             look_at = (b.look_at == true),
             color = b.color or { 200, 200, 200 },
+            texture = b.texture,             -- optional atlas-id (string) or nil
+            z_order = b.z_order or 0,        -- render-sort key; default 0
+            scale = b.scale or { 1.0, 1.0 }, -- [sx, sy]; negative = mirror
+            anchor = b.anchor,               -- optional [x, y] override of atlas-anchor; resolved in load_textures
+            texture_handle = nil,            -- populated by load_textures
         }
     end
 
@@ -110,12 +115,21 @@ function M.build_rig(rig_table)
         tracks_by_id[t.id] = { id = t.id, bones = bones_in_track }
     end
 
+    -- Build z-sorted bone list (lower z_order first = back, higher = front).
+    local bones_z_sorted = {}
+    for _, b_src in ipairs(rig_table.bones) do
+        bones_z_sorted[#bones_z_sorted + 1] = bones_by_id[b_src.id]
+    end
+    table.sort(bones_z_sorted, function(a, b) return a.z_order < b.z_order end)
+
     return {
         id = rig_table.id or "anonymous",
+        asset_pack = rig_table.asset_pack,   -- optional alias-key into module's asset_aliases
         bones = rig_table.bones,           -- keep original-order array
         bones_by_id = bones_by_id,
         tracks_by_id = tracks_by_id,
         bone_to_track = bone_to_track,
+        bones_z_sorted = bones_z_sorted,
     }
 end