fix(render3d, runtime): a glTF document is freed whole, layouts found by number, block records reused

- Json.free_all (value_free_all): a parsed tree's nodes, lists and strings. render3d frees each
  glTF document that way at the next load; the names kept out of it are copies (a primitive's
  material, a skin's joints, an animation clip's name in ludic.anim) - a model's strings were
  ~640 KB left behind per load
- jp_number made a digits list per decimal in a document and never freed it
- gvk_layout_id matches a mesh's layout as numbers in a scratch made once, against the layouts
  known end to end; a new mesh no longer builds a key string
- gvk_mem_new puts a new block into the record of one given back rather than appending, so a
  buffer made again every few frames no longer grows the block lists

steady.ludic adds a glTF parsed and freed whole 200 times: 0 bytes (38,400 before the digits fix),
beside the buffer path and the frame, still 0. A model loaded and let go still keeps ~2 KB a round
(texture and buffer handles are not reused yet); it is bounded at 8 KB a round.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.render3d/skin.ludic

@@ -87,7 +87,8 @@ function skin_load(render3d_st: mut Render3dState, idx: int) -> Skin {
   for i in 0 .. n {
     let nd = value_at(nodes, i)
     var nm: string = ""
-    if value_has(nd, "name") != 0 { nm = value_as_str(value_get(nd, "name")) }
+    # a copy: the document goes, strings and all, at the next load
+    if value_has(nd, "name") != 0 { let s = value_as_str(value_get(nd, "name"));  nm = s[0 .. len(s)] }
     push(sk.names, nm)
     skin_jv3(sk.rest_t, i * 3, nd, "translation", 0.0, 0.0, 0.0)
     skin_jv3(sk.rest_s, i * 3, nd, "scale", 1.0, 1.0, 1.0)