feat(render3d): replace the world at run time, water bodies, terrain_sea

terrain_reload/terrain_unload release one map's height field, survey, photograph
and patch bounds and generate another at any TERRAIN_HALF; scatter_clear_all
(with streams), actor_clear_all, col_reset and mesh_free empty the scene;
water_body_add/water_bodies_clear draw several still-water planes with one
reflecting; terrain_sea separates the coast's sea level from the carved lake's,
and grass keeps off both. Fixes CDLOD patch bounds for TERRAIN_HALF != 4096 and
water_init leaking a mesh and program per call. examples/rendering/reload.ludic.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

examples/rendering/reload.ludic

@@ -0,0 +1,82 @@
+# reload.ludic — the world replaced at run time.
+#
+# Generate a 2 km map, scatter onto it, give it colliders and water; release all of it and
+# generate a 3 km map in its place with two water planes; then go back. Prints RELOAD OK when
+# nothing of the previous map survived and the new one is whole.
+#
+#   bin/ludic build examples/rendering/reload.ludic --headless && ./build/reload_headless
+program Reload {
+  import "ludic.render3d/r3d.ludic"
+
+  property Marker { on: int = 1 }
+  model Anchor { Marker }
+
+  # A program only links the GL runtime render3d stands on when it names Gl.* itself
+  # (the parser's g_uses_gl), so even a check that draws nothing presents its frame.
+  # render3d calls back into the program for what stands on the terrain
+  function scene_draw() -> void { scatter_draw() }
+  function scene_draw_casters() -> void { scatter_draw_casters(shadow_cascade_vp(sh_cascade)) }
+  # no streamed cover here, but stream.ludic names the generator, so every program supplies one
+  function stream_fill(s: Stream, cx: int, cz: int, band: int) -> void { }
+
+  handler Present phase Render {
+    Gl.swap()
+  }
+
+  function check(what: string, cond: bool) -> bool {
+    if not cond { print(`reload: FAILED - {what}`) }
+    return cond
+  }
+  function populate() -> void {
+    let l = layer_cards(model_cross_card(), 1000, F_ZERO, fi(400))
+    for i in 0 .. 400 {
+      let x = fi(i * 2 - 400)
+      layer_add(l, x, terrain_height(x, fi(30)), fi(30), F_ONE, F_ZERO, F_ZERO, F_ZERO)
+      col_add(x, fi(30), F_ONE)
+    }
+    col_build()
+  }
+
+  handler Boot phase Start {
+    spawn Anchor {}
+    TERRAIN_HALF = 1000
+    ter_smooth = true
+    if not r3d_init(640, 360, "Reload") { quit() }
+    cam_set(F_ZERO, fi(80), fi(300), fi(180), f_neg(fi(10)))
+    water_init(fl(4.0), F_ZERO, F_ZERO, fi(200), fi(200))
+    populate()
+    var ok = true
+    ok = check("the first map has layers and colliders", len(sc_layers) == 1 and col_n == 400) and ok
+    # --- swap to a 3 km map with a sea and a lake above it
+    scatter_clear_all()
+    actor_clear_all()
+    col_reset()
+    water_bodies_clear()
+    terrain_lake(F_ZERO, F_ZERO, F_ZERO, F_ZERO, F_ZERO)
+    terrain_reload("", F_ZERO, F_ZERO, F_ZERO, F_ZERO, F_ZERO, "", 1500)
+    ok = check("everything of the first map released", len(sc_layers) == 0 and col_n == 0 and wb_n == 0 and not water_on) and ok
+    ok = check("the new half", TERRAIN_HALF == 1500 and ter_heights != null and ter_height_tex != 0) and ok
+    ok = check("patch bounds rebuilt", cd_min != null and len(cd_min) == CD_LEVELS) and ok
+    ok = check("patches scale with the map", cd_size(0) == f_mul(fi(32), fr(3000, 8192))) and ok
+    terrain_height(F_ZERO, F_ZERO)
+    ok = check("the cached height scale is the new map's", ter_h_scale == fr(TERRAIN_RES, 3000)) and ok
+    let sea = water_body_add(F_ZERO, F_ZERO, F_ZERO, fi(20000), fi(20000), true)
+    let lake = water_body_add(fl(12.0), fi(600), fi(-400), fi(150), fi(100), false)
+    ok = check("two water bodies, the sea mirrored", sea == 0 and lake == 1 and wb_n == 2 and wb_primary == 0 and water_level == F_ZERO and water_on) and ok
+    populate()
+    ok = check("the new map takes layers and colliders", len(sc_layers) == 1 and col_n == 400 and col_side == 3000 / COL_CELL) and ok
+    # a sea below the carved lake: set once, and not undone by a reload
+    terrain_sea(fl(-2.0))
+    terrain_lake(fl(12.0), fi(600), fi(-400), fi(150), fi(100))
+    ok = check("the sea is its own level", ter_sea_set and ter_sea_gen() == fl(-2.0) and ter_lake_level == fl(12.0)) and ok
+    # --- and back: the swap is repeatable
+    scatter_clear_all()
+    col_reset()
+    water_init(fl(4.0), F_ZERO, F_ZERO, fi(200), fi(200))
+    terrain_reload("", F_ZERO, F_ZERO, F_ZERO, F_ZERO, F_ZERO, "", 1000)
+    populate()
+    ok = check("back to 2 km", TERRAIN_HALF == 1000 and len(cd_min) == CD_LEVELS and wb_n == 1 and col_side == 2000 / COL_CELL) and ok
+    if ok { print("RELOAD OK") } else { print("RELOAD FAILED") }
+    quit()
+  }
+}