Three fixes close the loop against the captured region file: the giant
roll precedes the template draw, surfaceY is the topmost non-air block
(getHeight already returns one past it), and structure biome refs
resolve through has_structure/ tags to the extracted tag files. The
weighted variant pick walks a fresh per-attempt GenerationContext
stream while failed entries drop from the pick stream without reseeding.
TestRuinedPortalStubMatchesVanillaStart pins chunk (1,0) to vanilla's
own saved start: portal_6, CLOCKWISE_90, mirror NONE, air pocket,
template position (16,12,0) — and asserts the neighbouring fixture
chunks stay portal-free.
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.
