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.
