render3d: the stream kinds' 7-byte baked record, and fixed layers held as the baked chunks that are in

sb7_pack / sb7_field are the one pack and unpack of the stream kinds' record (56 bits, least significant
first: x 12, z 12, y 12 over the chunk's y span, scale 6, yaw 6, seed 4, wind 4, read as a low and a high
word), which the bake and the game both import; stream_emit_baked decodes a prefix of it.
layer_chunks_begin / layer_chunk_put / layer_chunk_drop keep a fixed layer (trees, boulders, stones:
12-byte records) as a slab per resident chunk in its instance list, an n x n index made at load, the
list re-uploaded at most once a frame. Nothing calls them yet: behaviour unchanged.

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

packages/ludic.render3d/scatter.ludic

@@ -39,6 +39,8 @@ property Layer {
   fog_buf:  int = 0,       # the casters inside the fog wall (fog_casters.ludic)
   fog_n:    int = 0,
   fog_src:  int = -1,
+  ck_off:   words,             # per baked chunk: where its slab starts in inst, or -1 (layer_chunks.ludic)
+  ck_cnt:   words,
   fog_all:  bool = false,  # the wall keeps (nearly) all of it: casters from sh_buf, no rebuild
   fog_wall: float = 0.0,
   fog_x:    float = 0.0,