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leonarski_fandClaude Opus 5 2fdc4d3bdf Post-refinement: the distance hypothesis is decided at the fit, on the excitation residual

Whether a sweep determines its detector distance at all was decided by running the canonical pass
twice - once at the post-refined distance, once at the header's - and keeping whichever realised the
lower held-out residual, the header's on a tie. The first-stage test that sent a run down that arm
compared the free and held fits on the POOLED held-out residual, and pooling is what made it
uninformative: the positional family cannot see the distance along the degenerate direction, so it
dilutes the one family that can, while the unpaired standard error of a heavy-tailed mean of squares
is 2-25 % of the mean against in-fit differences of 0.1-1 %. The test therefore said "cannot tell" on
four fifths of the fitted sweeps and the arm ran on most of a corpus, buying by prior what it could
not measure.

The two hypotheses are now compared on the one residual family that can tell them apart. The
excitation residual never involves the detector, so it is blind to the distance itself; what it sees
is the cell scale, and a held fit at a wrong header distance is forced into a wrong cell scale by the
spot positions, which the rocking angles then refuse - measured on a sweep whose header was 1.4 %
long, the held fit's held-out excitation residual is seventeen times the free fit's. Where freeing
the distance lowers that residual by more than its own standard error the free fit is committed as
before; where it does not, the held fit is committed - header distance, refined beam, cell,
orientation and axis - and the walk and the commit run held. The positional residual is deliberately
not consulted: its in-fit gain along the degenerate direction is the one re-integration erases.

The question is asked only while the run is still at the file's distance. A run that has walked off
the header has already refuted that hypothesis by re-integrating, and asking it again at every round
stalls a walk short of its fixed point, because the excitation standard error at the walk's tail is
outlier-dominated (measured: a walk stopped 0.4 % early, seven passes, cell 0.65 % off against
0.26 %, ISa 10.6 against 14.0).

So there is no arm, and with it go the two probe passes that measured it and the canonical pass
the losing arm used to cost: the decision costs two Ceres solves. On thirteen
rotation sweeps covering both verdicts, every decision the arm took by evidence or by its tie rule is
reproduced at the fit, except where the excitation family sees what the pooled test could not and the
free distance - the better cell against an external reference - is taken instead; merged intensities
are unchanged where the verdict is.

POSTREFINE_DISTANCE_HELD now means "the committed fit held the header distance", and is cleared where
a later geometry walk left it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nW6FNRP1bBJJ8pfHiByAT

2026-09-20 18:45:18 +02:00