render3d: ground layers placed from painted densities - the density tiles (LGD1), their reader and the fill

ground_density_bake(path, key, version, hash, pngs) cuts each layer's PNG (R the chance a cell keeps one,
G an optional scale) into the terrain's 64 m tiles, one layer decoded and let go at a time, as a bake's
file; ground_density_open(...) reads it - or, when it is missing or stale, a dev build's own copy cut from
the PNGs - a tile at a time into a 256-tile clock, never a layer whole. ground_fill(s, cx, cz, band, g)
places a GroundFill row's instances in a stream chunk: a candidate per cell of the band's step, jittered,
kept when a hash of (layer, chunk, band, cell) falls under the density (times the game's trample,
r3d_on_ground_trample, where the row asks); scale, yaw, seed and wind from the same hash. Nothing calls
it yet: behaviour unchanged.

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

packages/ludic.render3d/env.ludic

@@ -837,6 +837,18 @@ export state Render3dState {
   tb_version: int = 0
   tb_shadow_ok: bool = false         # the shadow came from its bake this map
   gltf_pure: int = 0                # a 1x1 white a factor multiplies exactly (gltf_factors)
+  gd_file: pointer = null            # the map's ground densities (ground_density.ludic)
+  gd_base: int = 0
+  gd_n: int = 0
+  gd_layers: int = 0
+  gd_ch: words = null
+  gd_off: words = null
+  gd_pool: []byte = null
+  gd_key: words = null
+  gd_ref: words = null
+  gd_slot_of: words = null
+  gd_hand: int = 0
+  gd_trample_cb: fn(float, float) -> float = null
   tt_dir: string = ""               # terrain_tiles_to: where a map's tiles are written ("" keeps the whole copies)
   tt_key: string = ""
   tt_file: pointer = null           # the tiles' file, open for reading (terrain_tiles.ludic)