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/Cargo.toml

@@ -7,6 +7,11 @@ license.workspace = true
 
 [dependencies]
 dr-types.workspace = true
+# The merge's geometry (FR-MRG-10): rotations, the focal length and the
+# projections, solved on proxies by dr-pano and consumed here per chunk. The
+# geometry alone — no keypoint model, no runtime — which is what the
+# workspace entry turns off.
+dr-pano.workspace = true
 dr-decode.workspace = true
 dr-pipeline.workspace = true
 # The watershed's pixel passes are here because they are shaders; everything