dr-gpu: the merge pass — warp, accumulate, resolve, chunk by chunk

merge.wgsl warps one camera-space tile into one output chunk — output
pixel to direction (the projection maths of dr_pano::projection, verbatim),
direction to the frame's camera, camera to source pixel, bilinear by hand
from four textureLoads because rgba32float is not filterable — and adds it
into a storage-buffer accumulator weighted by its distance from the
frame's edge. A resolve pass divides by the weights and packs sixteen-bit
samples at the sensor's scale with a coverage bit.

MergePass::merge drives it: bands of rows, chunks across a band, and for
each chunk only the frames whose footprint meets it, each rendered as the
source rectangle the chunk needs and nothing more. The working set is one
chunk, one tile and one band (FR-MRG-11); the frame textures are the
caller's to cache. Feathered, not seamed; gain a scalar per frame — the
blend quality is panorama.md §10's step 5, after the path writes a file.

core/dr-gpu/src/lib.rs

@@ -28,6 +28,7 @@ mod error;
 mod focus;
 mod histogram;
 mod mask;
+mod merge;
 mod raw_histogram;
 mod readback;
 mod segment;
@@ -37,6 +38,7 @@ pub use adjust::AdjustPass;
 // unclipped, full internal precision (FR-DEV-2), and anyone reasoning about
 // VRAM at 24 MP needs to know what an intermediate costs.
 pub use demosaic::{DemosaicedImage, Demosaicer};
+pub use merge::{Band, MergeFrame, MergeOutput, MergePass};
 pub use detail::INTERMEDIATE_FORMAT as DETAIL_INTERMEDIATE_FORMAT;
 pub use error::GpuError;
 pub use focus::{FocusPeakPass, FocusPeaking, PeakColour, PeakSensitivity};