dr-pano: a second XFeat shape for portrait frames, and a matcher that takes seconds

Twelve real frames from the fixture set now align in 4.5 s — 4.4 s of
matching, 118 ms of bundle adjustment — where the first run took 51 s and
left the first two frames out.

The matcher computes each pair's similarity matrix once, across the
cores, with a dot product written to vectorise; both nearest-neighbour
directions read it. The frames that failed were portrait: fitted into the
landscape input they used 512 of 1024 px, and their thin overlap did not
survive at half resolution. The same weights are now exported at 768×1024
as well and the detector picks the shape by aspect. The example aligns
from embedded previews and draws the set on a cylinder; on the fixture the
sweep is 152° at a fitted 47.9 mm against the EXIF's 50, RMS 1.5 px, and
the overlaps show no ghosting.

models/LICENCE.md

@@ -76,10 +76,13 @@ rather than a code change — as this document predicted it would be.
 
 | File | Source | Trained on | Used by |
 |---|---|---|---|
-| `keypoints/xfeat-1024.onnx` | `weights/xfeat.pt` from `https://github.com/verlab/accelerated_features` | MegaDepth + synthetic warps, by the authors | panorama alignment (FR-MRG-8) |
+| `keypoints/xfeat-1024.onnx` | `weights/xfeat.pt` from `https://github.com/verlab/accelerated_features` | MegaDepth + synthetic warps, by the authors | panorama alignment (FR-MRG-8), landscape frames |
+| `keypoints/xfeat-768.onnx` | the same weights | — | the same, portrait frames |
 
-Exported by `tools/export-xfeat.sh` at a fixed 768×1024 grayscale input.
-Only the convolutional network is in the file; the keypoint decoding is Rust.
+Exported by `tools/export-xfeat.sh` at fixed grayscale inputs of 1024×768
+and 768×1024 — the same weights twice, because tract needs a static shape
+and a portrait frame in a landscape input wastes half of it. Only the
+convolutional network is in each file; the keypoint decoding is Rust.
 
 **The repository and its weights are Apache-2.0**, read on 2026-09-19 from the
 `LICENSE` at its root, with no separate grant on the checkpoint and no