Bind the surface rule tree to the world seed

The tree was parsed once, globally, and shared by every world -- so every
condition that needs the seed simply did not work. Compiling it per RandomState
fixes four of them at once.

noise_threshold sampled a per-column random draw and pretended it was
"minecraft:surface"; the other six noises it names were unsupported and returned
false. Each condition now holds its own seeded noise, sampled once per column
into a small cache the way vanilla's LazyXZCondition does. Powder snow, packed
ice and ice appear in the dump for the first time; calcite, swamp water windows
and gravel patches have their conditions back too.

vertical_gradient tapered through a per-column RNG shared with the other rules.
Vanilla rolls a positional random at the exact block, from a factory named by
the rule. More importantly the anchor decoder read only above_bottom and
discarded which kind of anchor it was, so the deepslate rule's absolute 0..8
collapsed onto y=-64 and **no deepslate existed anywhere in the world**. Anchors
now carry their kind and resolve against the real height bounds -- which also
retires a hardcoded 384 in y_above.

Two more stubs land with them: hole is surfaceDepth <= 0 rather than a constant
false, and steep reads the neighbouring column heights. steep needs the whole
chunk's heightmap, so the column pass is now two passes -- terrain and fluids
for all 256 columns, then surface rules -- which is the order vanilla uses
anyway (doFill, then buildSurface).

Deepslate was also missing from the block-ID table, and an unknown name resolved
to 0, which the caller read as "no block" and skipped. So even a correct rule
would have placed nothing. Unknown names are now a parse error, deepslate and
mud are in the table, and a rule that resolves to air genuinely places air --
the frozen-ocean surface asks for exactly that.

Below y=0 is now entirely deepslate, y=1..7 a scatter, above y=8 none.

internal/worldgen/blockids.go

@@ -20,80 +20,97 @@ package worldgen
 
 // surfaceBlockID resolves a surface-rule result_state (Name + optional
 // Properties) to its network block-state ID. It handles the snowy property on
-// snowable blocks and the layers property on snow; unknown blocks return 0
-// (air) so a missing entry is visually obvious rather than crashing.
-func surfaceBlockID(name string, props map[string]string) uint16 {
+// snowable blocks and the layers property on snow.
+//
+// An unknown name is an error, not a fallback. It used to return 0, which the
+// rule application then read as "no block" and skipped — so a name missing from
+// this table silently left stone behind. That is exactly how deepslate went
+// missing from the entire world: the rule fired, resolved to 0, and was dropped.
+func surfaceBlockID(name string, props map[string]string) (uint16, bool) {
 	switch name {
+	case "minecraft:air":
+		return 0, true
+	case "minecraft:deepslate":
+		// A pillar block; the surface rule always asks for the upright axis.
+		return 27924, true
+	case "minecraft:mud":
+		return 27922, true
+	case "minecraft:brown_terracotta":
+		return 11456, true
+	case "minecraft:red_terracotta":
+		return 11458, true
+	case "minecraft:light_gray_terracotta":
+		return 11452, true
 	case "minecraft:stone":
-		return 1
+		return 1, true
 	case "minecraft:granite":
-		return 2
+		return 2, true
 	case "minecraft:diorite":
-		return 4
+		return 4, true
 	case "minecraft:andesite":
-		return 6
+		return 6, true
 	case "minecraft:grass_block":
 		if props["snowy"] == "true" {
-			return 8
+			return 8, true
 		}
-		return 9
+		return 9, true
 	case "minecraft:dirt":
-		return 10
+		return 10, true
 	case "minecraft:coarse_dirt":
-		return 11
+		return 11, true
 	case "minecraft:podzol":
 		if props["snowy"] == "true" {
-			return 12
+			return 12, true
 		}
-		return 13
+		return 13, true
 	case "minecraft:bedrock":
-		return 85
+		return 85, true
 	case "minecraft:water":
-		return 86
+		return 86, true
 	case "minecraft:sand":
-		return 118
+		return 118, true
 	case "minecraft:red_sand":
-		return 123
+		return 123, true
 	case "minecraft:gravel":
-		return 124
+		return 124, true
 	case "minecraft:sandstone":
-		return 578
+		return 578, true
 	case "minecraft:red_sandstone":
-		return 13247
+		return 13247, true
 	case "minecraft:snow_block":
-		return 6928
+		return 6928, true
 	case "minecraft:snow":
 		// snow has a "layers" property 1..8; default layer 1 = 6919.
-		return 6919
+		return 6919, true
 	case "minecraft:ice":
-		return 6927
+		return 6927, true
 	case "minecraft:packed_ice":
-		return 12914
+		return 12914, true
 	case "minecraft:powder_snow":
-		return 24689
+		return 24689, true
 	case "minecraft:mycelium":
 		if props["snowy"] == "true" {
-			return 8918
+			return 8918, true
 		}
-		return 8919
+		return 8919, true
 	case "minecraft:terracotta":
-		return 12912
+		return 12912, true
 	case "minecraft:white_terracotta":
-		return 11444
+		return 11444, true
 	case "minecraft:orange_terracotta":
-		return 11445
+		return 11445, true
 	case "minecraft:yellow_terracotta":
-		return 11448
+		return 11448, true
 	case "minecraft:calcite":
-		return 24687
+		return 24687, true
 	case "minecraft:tuff":
-		return 23452
+		return 23452, true
 	case "minecraft:dripstone_block":
-		return 27755
+		return 27755, true
 	case "minecraft:moss_block":
-		return 27862
+		return 27862, true
 	case "minecraft:smooth_stone":
-		return 13480
+		return 13480, true
 	}
-	return 0
+	return 0, false
 }