render3d: ground_fill's candidate is a function of its own; solid layers with stable ids; the trample as clearing discs

ground_candidate answers (layer, chunk, band, cell) -> keep, x, z, scale, yaw, seed, wind from pure gf_*
steps and the density read between them; ground_fill draws its answers with the same operations in the
same order. A solid layer fills at step0 and band 0 whatever the camera, a far band drawing a stable
subset (draw 7 under (step0/step_b)^2), each thing an int id from (layer, chunk, cell), listed per chunk
into a caller's GroundSolids or answered by id. r3d_ground_clearing hands the trample over as discs.

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

packages/ludic.render3d/env.ludic

@@ -860,6 +860,9 @@ export state Render3dState {
   gd_zbuf: []byte = null             # one tile's deflated bytes as read
   gd_hand: int = 0
   gd_trample_cb: fn(float, float) -> float = null
+  gd_clear: floats = floats(GD_CLEAR_MAX * 5)   # the clearings as data: (x, z, r_in, r_out, floor) each (ground_clear.ludic)
+  gd_clear_n: int = 0
+  gd_cand: GroundCand = new GroundCand   # ground_fill's candidate, filled and reused
   gb_layer: int = -1                 # the densities' layer the blades grow by; -1: the rules (grass_density.ludic)
   gb_buf: int = 0                    # its window round the camera, on the GPU
   gb_win: words = null               # ... and the window's bytes, four texels a word, made once