world: port ruined portal generation point and expose pre-carve terrain

generateBaseTerrain now factors the noise+surface stage out of chunk
generation so structure placement can read pre-carve heights exactly
like vanilla's STRUCTURE_STARTS stage. RuinedPortalGenerationPoint
replays the ruined_portals set: the weighted pick across all seven
variants on its own stream, each attempt's setup/template/rotation/
mirror draws on a fresh GenerationContext stream (makeRandom reseeds
per attempt, confirmed from bytecode), findSuitableY with corner-column
settle scan against those heights, and the post-draw 3D-biome filter at
quart-snapped coordinates.

Known gap, documented in STRUCTURE_NOTES.md: no variant is accepted in
the window around the fixture's portal yet while vanilla placed one;
the biome/surface inputs need a vanilla-side probe to bisect. Not wired
into the production replay until that closes.

internal/worldgen/biome_temperature.go

@@ -97,3 +97,7 @@ func coldEnoughToSnow(biome string) bool {
 	temperature, ok := biomeTemperature[biome]
 	return ok && temperature < 0.15
 }
+
+// ColdEnoughToSnow exposes the per-biome snow threshold to structure ports.
+func ColdEnoughToSnow(biome string) bool { return coldEnoughToSnow(biome) }
+