Serial stills: the indexing ambiguity resolved from the data when no reference or model is given

Where the space group is given (-S) and its lattice admits alternative indexings, a stills run without
-z or --model used to merge the crystals' random indexings together (and warned). The crystals are now
made consistent with each other before anything is scaled (ReferenceFreeReindex): each takes the
indexing in which it agrees best with all the others (Brehm & Diederichs 2014), agreement being the
sum over shared reflections of the products of shell-normalised, per-crystal standardised intensities,
weighted by 1/(1+(sigma/sd)^2). The sum over all pairs is (|M|^2 - const)/2 for M the running sum, so a
pass over the crystals is one pass over the reflections rather than over all pairs: coordinate ascent,
four seeded starts, stop when <= 1 crystal in 1000 moves, the overall relabel that moves the fewest
crystals. Deterministic (fixed summation order, sequential passes).

A twin law of a pseudo-merohedral cell whose axes differ by a few per cent is decided by the crystals'
own refined cells, which the indexer has already used: where >= 90% of the crystals' cells fit better as
indexed, those crystals are anchors and only the others are decided by the intensities. Without this,
two public sets got worse (pal90 R-free 0.183 -> 0.207, lcls242_t 0.296 -> 0.350).

With a reference or model the per-image choice stands; the same resolution runs as a check, reported as
AMBIGUITY_DATA_DISAGREES. New report keys (section 2): INDEXING_AMBIGUITY_RESOLVED, AMBIGUITY_CRYSTALS,
AMBIGUITY_REINDEXED, AMBIGUITY_SEPARATION_SIGMA, AMBIGUITY_DECIDED_FRACTION (+ AMBIGUITY_TIME_S, dev).
--mode scale is not changed.

Still battery, no --model (R-free, REFMAC 0-cycle against the deposited model; before -> after; with
--model for comparison): euxfel83 0.321 -> 0.207 (0.204), euxfel152 0.346 -> 0.257 (0.245), alvra213
0.209 -> 0.201 (0.207), alvra213_int16 0.210 -> 0.202 (0.208), pal90 0.183 -> 0.183 (0.188), lcls242_t
0.296 -> 0.294 (0.285); controls sacla237 / lcls222 unchanged (no ambiguity, no-op). The step takes
0.2-1.6 s on 2.7k-54k crystals; synthetic 64k crystals x 200 reflections 0.5 s.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015CeEGgpKdfkCTJb3YNMJcu
(cherry picked from commit 80d378a1fe92364fd5b99a17f5c9ee8498412d36)

docs/ACKNOWLEDGEMENT.md

@@ -367,6 +367,16 @@ S. C. Abrahams, H. D. Flack, E. Prince and A. J. C. Wilson, "Statistical descrip
 crystallography. II. Report of a Working Group on Expression of Uncertainty in Measurement" (1995),
 Acta Cryst. A51, 565-569 [doi:10.1107/S0108767395002340](https://doi.org/10.1107/S0108767395002340).
 
+**Indexing ambiguity of serial data without a reference** — where a serial run's lattice is more
+symmetric than its crystals and no reference or model is given, each crystal is put into the indexing in
+which its intensities agree best with those of all the other crystals, crystal by crystal until none
+changes. The objective - the summed agreement between every pair of crystals - and the iteration that
+maximises it are taken from Brehm and Diederichs, whose method CrystFEL's `ambigator` also implements;
+rugnux measures the agreement as a product of standardised intensities rather than a correlation, which
+lets the sum over all partners be taken against one running sum instead of pair by pair.
+W. Brehm and K. Diederichs, "Breaking the indexing ambiguity in serial crystallography" (2014),
+Acta Cryst. D70, 101-109 [doi:10.1107/S1399004713025431](https://doi.org/10.1107/S1399004713025431).
+
 ### Physical corrections and calibration
 
 **Sensor absorption at oblique incidence, and the flight path** — the angle-dependent quantum