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>

image_analysis/bragg_integration/BraggIntegrationEngineGPU.h

@@ -45,6 +45,9 @@ class BraggIntegrationEngineGPU : public BraggIntegrationEngine {
     // The learning/fit math is single precision: FP64 is heavily throttled on consumer GPUs and the
     // extraction is Poisson-noise limited, so float reproduces the double CPU path to ~1e-4.
     CudaDevicePtr<uint8_t> d_mask;                       // per-pixel inner-stencil reflection mask
+    // Per-pixel (distance, reflection) key naming the nearest predicted centre; allocated only when
+    // an overlap treatment is on, so the default path costs no extra device memory.
+    CudaDevicePtr<uint32_t> d_owner;
     CudaDevicePtr<float>   d_shell_grid, d_global_grid;  // learned profile accumulators (N_SHELL*GG, GG)
     CudaDevicePtr<float>   d_shell_P, d_global_P;         // normalised profiles (empirical mode)
     CudaDevicePtr<float>   d_mom;                         // learned 2nd moments, 3 per shell + global