Bragg integration: a shared signal pixel belongs to the nearer reflection

Nothing kept a neighbour's flux out of a reflection's own signal disk. The union mask
keeps neighbour cores out of the BACKGROUND ring, but the r1 disk was read whole, so on
a dense pattern a crowded reflection measures part of its neighbour as its own.

Ownership is decided once per image into a per-pixel (quantised distance, reflection)
key written with an atomic minimum, so the nearest predicted centre wins whatever order
the writes arrive in and the lowest index breaks a tie. `--overlap exclude`, now the
default, drops the pixels a nearer neighbour owns from the profile fit. A profile fit is
the amplitude of a normalised profile, so leaving pixels out renormalises the estimator
by construction and the reflection stays unbiased rather than being discarded; the
summation-fallback guard is scaled back to the disk the box-sum seed actually read, so
it still compares like with like. `--overlap reject` is the XDS MINPK alternative - drop
the reflection when less than `--overlap-minpk` of its expected profile is cleanly its
own. A box sum has no profile to renormalise with, so `exclude` is a no-op there and
only `reject` acts on it.

Widening the split - keeping a pixel only where no other centre is within its distance
PLUS a margin - was built and measured, and it is worse monotonically: the residual bias
of the pixels that were kept grows from +0.072 to +0.209 in ln intensity at 0 to 3 px of
margin. What the margin removes is the reflection's own profile, not the neighbour's
tail, so the plain nearest-centre split is the rule.

Measured on the full 38-crystal rotation battery against the same binary with the
treatment off: ISa better 15 / worse 8, summed shortfall against XDS 39.7 -> 28.1. Three
of the losses are the two-pass loop taking its other branch - their median mosaicity
moves between the two known attractors - rather than the change under test; excluding
those it is better 15 / worse 5 and the shortfall goes 31.3 -> 14.4. The two crowded
crystals gain 38% and 52% of their ISa, one of them passing XDS. High-shell CC1/2 over
the 35 crystals that neither flipped branch nor carry a collapsed error model is better
7 / worse 7. Space groups unchanged at 35/38. The owner map is built only when a
treatment is asked for and costs 1.1% of the battery's wall clock - 23% on a genuinely
crowded crystal, nothing where no two predictions touch.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/CHANGELOG.md

@@ -5,6 +5,7 @@ This is an UNSTABLE release. It includes many experimental features, as well as
 
 * rugnux: The error-model **a** and **b** are reported in XDS's convention, and `_reflns.jfjoch_diffrn_ISa` now carries the whole-range `1/sqrt(a*b)` that XDS's ISa denotes; the strong-reflection asymptote moves to `_reflns.jfjoch_diffrn_ISa_asymptotic`. **A file written by an earlier version carries the asymptote under the old name.**
 * Bragg integration: the background ring's outer radius default changes from 10 px to **13 px**, which roughly doubles the pixels behind each background estimate; the signal disk is unchanged.
+* Bragg integration: signal pixels shared with a neighbouring reflection are now dropped from the profile fit (`--overlap off|reject|exclude`, **default `exclude`**), so a crowded reflection no longer reads its neighbour's flux as its own; `reject` instead discards a reflection whose cleanly-observed profile fraction is below `--overlap-minpk` (default 0.75).
 * rugnux: The background ring can be **elongated radially per reflection** for broadband data (`--integration-stencil <k>`, default 0 = the fixed circular ring), by `k` times the beam's radial streak; the `r1` signal box stays circular.
 * rugnux: New **beam-stop shadow detection**, **on by default** (`--detect-beam-stop[=N|off]`), finds the beam stop and its holder in a projection of N images (default 60) and adds them to the pixel mask as bit 9, which is cleared at the start of every run.
 * Viewer: the detected beam-stop shadow is drawn in coral, with a "Show beam stop" switch in the side panel.