Bragg integration: drop the 2% sigma floor and carry the background variance

Two changes to the same variance chain; they are in one commit because the second
exists to remove an assumption the first was breaking, and separating them leaves a
tree that is correct only by luck.

The reported sigma was floored at 2% of the intensity, a per-partial I/sigma cap of
50. It applied only to the box-sum seed, never to the profile fit, so the shipped
default was unaffected - but the combine back-derives each partial's non-signal
variance as sigma^2 - I, and a floored sigma makes that quantity mean nothing. It
then read corr^2 * (0.0004 I^2 - I), which is not a background variance. Measured on
--integrator boxsum: the reported sigma understated the true scatter by up to 16x at
I ~ 21000 counts per partial, and pooled_I amplified a 1 ct/px background drift into
an 11.5% intensity error on the strongest reflections.

What the floor stood in for - that at high intensity the error is systematic rather
than counting - is already carried downstream, twice: the fitted b in
v = a*sigma^2 + (b*I)^2, measured from the data rather than assumed, and
SigmaWithSystematicFloor on the merged sigma. The floor was that idea applied one
level too early with a hardcoded b of 0.02. It arrived without a test or a setter and
was unreachable from the CLI, the API and the config.

The merge now takes the non-signal variance the integrator actually measured instead
of inverting sigma^2 = I + N. That identity is exact for a box sum once the floor is
gone and was never exact for a profile fit, whose sigma^2 = 1/den + (wsum/den)^2 *
bkg_var is formed against a fitted intensity. The value is carried through
BraggFitResult, Reflection and Obs, both engines, both merges, and the process-file
round trip; files written before this change are read with the term absent, which is
what they had.

Battery, 37 crystals, paired: space groups unchanged, reflection sets unchanged,
median delta zero on R_meas and CC1/2. --integrator boxsum on the reference crystal
goes ISa 8.9 -> 20.2 with a 0.947 -> 1.032.

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

image_analysis/bragg_integration/BraggIntegrationEngineGPU.h

@@ -29,7 +29,7 @@ class BraggIntegrationEngineGPU : public BraggIntegrationEngine {
     // --- per-reflection device arrays (grown by EnsureCapacity) ---
     CudaDevicePtr<float>   d_px_x, d_px_y, d_d;
     CudaDevicePtr<int>     d_cx, d_cy;
-    CudaDevicePtr<float>   d_I, d_sigma, d_bkg, d_bkg_var, d_obs_x, d_obs_y;
+    CudaDevicePtr<float>   d_I, d_sigma, d_bkg, d_bkg_var, d_var_bkg, d_obs_x, d_obs_y;
     CudaDevicePtr<float>   d_isum;          // box-sum raw sum, for the radial correction
     CudaDevicePtr<int>     d_ninner, d_rbin;
     CudaDevicePtr<uint8_t> d_ok, d_strong, d_has_obs;
@@ -51,7 +51,7 @@ class BraggIntegrationEngineGPU : public BraggIntegrationEngine {
 
     // --- host staging (copied back once per frame) ---
     std::vector<float>   h_px_x, h_px_y, h_d;
-    std::vector<float>   h_I, h_sigma, h_bkg, h_obs_x, h_obs_y;
+    std::vector<float>   h_I, h_sigma, h_bkg, h_var_bkg, h_obs_x, h_obs_y;
     std::vector<uint8_t> h_ok, h_has_obs;
 
     void EnsureCapacity(size_t n);