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.anim/read.ludic

@@ -62,7 +62,7 @@ function read_one(anim_st: AnimState, render3d_st: mut Render3dState, a: Val) ->
   }
   if n_keep == 0 { return null }
   let c = new Clip
-  c.name = nm
+  c.name = nm[0 .. len(nm)]              # a copy: the document goes, strings and all, at the next load
   c.n = n_keep
   c.cnode = words(n_keep)
   c.cpath = words(n_keep)