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>

packages/ludic.render3d/shaders/grass.vert

@@ -20,6 +20,8 @@ uniform float u_ts_half;
 uniform sampler2D u_ortho;
 uniform float u_ortho_on;
 uniform float u_lake_level;
+uniform float u_sea_level;    // the sea (terrain_sea); the lake's level when there is no separate sea
+uniform vec4 u_lake;          // the carved lake: centre x/z, half extents (z = 0: none)
 uniform float u_snow_line;
 uniform float u_wind;
 uniform vec2 u_tile;         // world xz of this tile's corner
@@ -92,7 +94,11 @@ void main() {
   if (croot.w < -1.0 || abs(croot.x) > croot.w * 1.25 + 1.5 || abs(croot.y) > croot.w * 1.4 + 1.5) { cull(); return; }
   vec3 gn = normalize(ht.gba);
   float h3 = bladeHash(ci, j, 2), h4 = bladeHash(ci, j, 3);
-  float ok = (1.0 - smoothstep(0.30, 0.55, 1.0 - gn.y)) * smoothstep(0.0, 0.6, ht.r - u_lake_level - 0.15) * smoothstep(u_snow_line - 80.0, u_snow_line - 200.0, ht.r);
+  // No blades under water: the sea's line everywhere, and the lake's inside its outline. One
+  // line for both left a lake above the sea with grass on its bed or a valley with none.
+  float wl = u_sea_level;
+  if (u_lake.z > 0.0) { vec2 q = (xz - u_lake.xy) / u_lake.zw;  if (dot(q, q) < 1.0) wl = max(wl, u_lake_level); }
+  float ok = (1.0 - smoothstep(0.30, 0.55, 1.0 - gn.y)) * smoothstep(0.0, 0.6, ht.r - wl - 0.15) * smoothstep(u_snow_line - 80.0, u_snow_line - 200.0, ht.r);
   if (u_ortho_on > 0.5) {
     vec3 oc = textureLod(u_ortho, huv, 1.5).rgb;
     ok *= 0.25 + 0.75 * smoothstep(0.0, 0.02, oc.g - max(oc.r, oc.b));

packages/ludic.render3d/shaders/heightgen.frag

@@ -10,6 +10,7 @@ uniform float u_dem_base;       // the elevation that becomes y = 0
 uniform vec2 u_origin;          // world x/z of the map's centre
 uniform vec4 u_lake;            // a lake: centre x/z, half extents (zero = none)
 uniform float u_lake_level;     // its surface height; the model records the surface, the bed is carved below it
+uniform float u_sea_level;      // the sea the coast falls into (terrain_sea); the lake's level unless set
 uniform vec4 u_isle;            // an island: centre x/z, radius (or rim margin), falloff width
 uniform float u_isle_mode;      // 0 none, 1 radial (terrain_island), 2 inward from the survey's edge (terrain_coast)
 // The DEM is Copernicus GLO-30 — 30 m data resampled onto this 4 m grid — so the stored
@@ -84,15 +85,15 @@ void main() {
       t = smoothstep(u_isle.z, u_isle.z + u_isle.w, d);          // 0 inland, 1 past the fall
       shelf = 26.0 + 34.0 * smoothstep(0.0, 1.0, (d - u_isle.z - u_isle.w) / max(u_isle.w, 1.0));
     }
-    float above = h - u_lake_level;
-    h = u_lake_level + above * (1.0 - t) - t * shelf;
+    float above = h - u_sea_level;
+    h = u_sea_level + above * (1.0 - t) - t * shelf;
     // A strand. Scaling alone hands a 500 m mountainside a 40-degree plunge into the sea,
     // which is a cliff coast and nothing else. Real shores are cut flat by the water they
     // meet, so the last few metres of height either side of the line are compressed —
     // which stretches them out horizontally into beach and shallows. Only inside the
     // coastal band: an inland lake has its own bed and wants none of this.
-    float near = 1.0 - smoothstep(0.0, 48.0, abs(h - u_lake_level));
-    h = mix(h, u_lake_level + (h - u_lake_level) * 0.30, near * smoothstep(0.02, 0.30, t));
+    float near = 1.0 - smoothstep(0.0, 48.0, abs(h - u_sea_level));
+    h = mix(h, u_sea_level + (h - u_sea_level) * 0.30, near * smoothstep(0.02, 0.30, t));
   }
   o = vec4(h, 0.0, 0.0, 1.0);
 #elif defined(SMOOTH)

packages/ludic.render3d/shaders/terrain.frag

@@ -18,6 +18,7 @@ uniform sampler2D u_snow_d;  uniform sampler2D u_carpet;   // the clump cards ba
 uniform float u_carpet_on;
 uniform float u_snow_line;
 uniform float u_lake_level;      // the ground just above the water is wet and dark
+uniform float u_sea_level;       // the sea's line: shore, forest and scree gates (terrain_sea)
 uniform vec2 u_origin;           // world offset of the terrain grid
 
 // The ground's own sun shadow comes from the baked height-field map (tershadow.frag),
@@ -236,14 +237,14 @@ vec3 groundShade(vec3 p, vec3 N, float slope, float dist, float viewDepth, bool
     vec3 ocf = textureLod(u_ortho, v_huv, 2.0).rgb;
     float gx = ocf.g - max(ocf.r, ocf.b);
     // (the photograph is sampled linear: sRGB 72 is 0.06, 92 is 0.11)
-    forestW = smoothstep(0.12, 0.06, max(ocf.r, max(ocf.g, ocf.b))) * smoothstep(0.004, 0.012, gx) * smoothstep(u_lake_level + 0.8, u_lake_level + 1.8, p.y);
+    forestW = smoothstep(0.12, 0.06, max(ocf.r, max(ocf.g, ocf.b))) * smoothstep(0.004, 0.012, gx) * smoothstep(u_sea_level + 0.8, u_sea_level + 1.8, p.y);
     // Classify from a ~60 m blur, never from the pixels: the survey's 10 m pixels carry
     // a foot trail as a broken line of bare ground, and thresholding them painted it
     // across the meadow as tan dashes (and, on the CPU, lined boulders up along it).
     vec3 ocl = textureLod(u_ortho, v_huv, 2.5).rgb;
     float mxc = max(ocl.r, max(ocl.g, ocl.b)), mnc = min(ocl.r, min(ocl.g, ocl.b));
     float greenEx = ocl.g - max(ocl.r, ocl.b);
-    screeC = smoothstep(0.008, -0.002, greenEx) * smoothstep(0.06, 0.12, mxc) * (1.0 - smoothstep(0.55, 0.75, mxc)) * smoothstep(u_lake_level + 0.2, u_lake_level + 1.2, p.y);
+    screeC = smoothstep(0.008, -0.002, greenEx) * smoothstep(0.06, 0.12, mxc) * (1.0 - smoothstep(0.55, 0.75, mxc)) * smoothstep(u_sea_level + 0.2, u_sea_level + 1.2, p.y);
     snowC = smoothstep(0.08, 0.04, mxc - mnc) * smoothstep(0.55, 0.8, mxc);
   }
   // snow lingering in the high gullies is drawn further down, but whether it can be
@@ -362,7 +363,7 @@ vec3 groundShade(vec3 p, vec3 N, float slope, float dist, float viewDepth, bool
   // the bank here: identical noise fields, identical time, identical phase, keyed off
   // height above the lake instead of depth below it. Across the seam the two agree, so
   // there is nothing there to read as an edge.
-  float above = p.y - u_lake_level;                       // >0 on land, metres
+  float above = p.y - u_sea_level;                        // >0 on land, metres
   // wet ground: darker and glossier near the water, fading out over ~1.2 m
   float wet = smoothstep(1.2, 0.0, above);
   alb *= mix(1.0, 0.5, wet * 0.85);