dr-pano: the geometry, from features to cameras

A new crate holding the CPU half of a merge (FR-MRG-10): the grayscale
proxy with orientation, the XFeat decoder ported step for step from the
reference detectAndCompute, mutual-nearest-neighbour matching, a robust
pairwise homography with the focal length read off it, a hand-rolled
Levenberg–Marquardt bundle adjustment over every rotation and the focal,
the three output projections, and align(), which chains it all and names
the frames it could not place rather than guessing (FR-MRG-5).

Dependency-free without the xfeat feature — linalg.rs says why the dense
algebra is hand-rolled — and tested on synthetic sweeps whose answer is
known exactly. The noise test records the single-row degeneracy: one
pixel of noise is a tenth of a percent of focal, which is a uniform
stretch of the sweep, not a misalignment.

Cargo.toml

@@ -11,6 +11,7 @@ members = [
     "core/dr-ingest",
     "core/dr-gpu",
     "core/dr-lens",
+    "core/dr-pano",
     "core/dr-pipeline",
     "core/dr-preset-xmp",
     "core/dr-segment",
@@ -48,6 +49,8 @@ dr-film = { path = "core/dr-film" }
 dr-ingest = { path = "core/dr-ingest" }
 dr-gpu = { path = "core/dr-gpu" }
 dr-lens = { path = "core/dr-lens" }
+# Optional runtime, like `dr-segment`: the geometry never needs a model.
+dr-pano = { path = "core/dr-pano", default-features = false }
 dr-pipeline = { path = "core/dr-pipeline" }
 dr-preset-xmp = { path = "core/dr-preset-xmp" }
 # `default-features = false` belongs *here*, not on each dependant: a member