Re-derived the other branch's central check with my own code, my own reading of
the record and my own control, rather than accepting the number.
At base chunk+0x10, with the plane at intra-record +0 and three u16 vertex
indices at +32: every one of 133573 faces has all three named vertices
satisfying its own plane equation. Random-vertex control 2782/400719 = 0.69%.
100% against 0.69% is not a fit.
That also settles the record boundary from my side: the u16s describing a plane
sit after it in the same 48-byte record, so my earlier 'four zeros at the start
of each record' was those integers seen 16 bytes out of position, one record
late.
Two implementations, two independent guesses at the intra-record layout, the
same 100%.
6f81ef93cf