appclip: camera detection of printed App Clip Codes

Ports the reference raster scanner (rs/appclipcode) to a grayscale
GrayFrame pipeline stage, wired into pipeline::scan_2d:

- Sobel edge field; strong edges keep their unit normals
- ring Hough: edge pixels vote along +-normal at each of the 11 expected
  ring-edge radii over an outer-radius sweep (on a <=320px copy), peaks
  hill-climbed at full resolution on an edge-vs-quiet-zone circle score
- affine/tilt estimate from the outer-circle edge cloud's second moments,
  refined by coordinate descent over centre, angle and both axis scales
- rotation search (the code has no fiducial): every 1 degree scored by the
  Otsu separability of per-position boundary contrast, best peaks refined
  to 0.1 degree
- bit recovery: gap/arc from thresholded contrast (Otsu + a sweep of
  plausible visible counts), 2-means over arc luminance for the
  foreground/third bit in both polarities, template-byte distance ordering
  attempts, Reed-Solomon arbitrating every one — first surviving URL wins

The port is deliberately color-blind: every Apple palette separates the
three colors by >=50/255 luminance, so grayscale sampling loses nothing.

One upstream deviation, deliberate: the Go scanner inserts a synthetic
separator bit and expects arc colors to start at ring bit 129 — consistent
only with its own synthetic-test renderer. Apple's actual generator (and
our SVG renderer, byte-verified against Apple fixtures) burns the
separator as the first visible arc's color, so the reconstruction here
appends sampled colors directly after the 128 gap bits.

Covered by generate -> rasterize -> scan round-trips (multiple sizes and
rotations, off-centre, gray surround), a scan_2d pipeline test, and a
no-circle rejection test; ANYD_APPCLIP_DEBUG dumps candidates and rotation
hypotheses. Busy no-circle crops cost ~100ms native / ~240ms wasm extra in
scan_all — same order as the QR fallback ladder and bounded by the
4-candidate cap; linear crops (decode1d) are unaffected.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: MagicalTux

README.md

@@ -44,7 +44,7 @@ Legend: ✅ done · 🚧 in progress · ⬜ planned.
 | Han Xin (versions 1–3, Numeric/Text/Byte) | ✅ | ✅ | ⬜ |
 | DotCode (GF(113) RS) | ✅ | ✅ | ⬜ |
 | Grid Matrix (all 13 versions) | ✅ | ✅ | ⬜ |
-| Apple App Clip Code (URL codec + SVG, camera & NFC variants)⁵ | ✅ | ✅ | ⬜ |
+| Apple App Clip Code (URL codec + SVG, camera & NFC variants)⁵ | ✅ | ✅ | ✅ |
 
 ### 2D — stacked
 
@@ -108,7 +108,10 @@ in `tests/pdf417_image.rs`.
 from-scratch implementation following the public reverse engineering of Apple's
 generator ([rs/appclipcode], MIT), byte-verified against `URLCompression.framework`
 output. Encode produces the circular five-ring SVG (all 18 Apple color templates,
-camera and NFC center glyphs); decode is structural (ring bits → URL). The trained
+camera and NFC center glyphs); decode covers both the structural path (ring bits →
+URL) and camera detection — a grayscale ring detector (gradient-normal Hough over the
+five-ring geometry, affine/tilt refinement, rotation search, RS-arbitrated bit
+recovery) wired into `pipeline::scan_2d`. The trained
 Huffman tables embed ~1.7 MB, so the module sits behind the default-on `appclip`
 cargo feature (`default-features = false` for a lean build). Apple, App Clips and
 related marks are trademarks of Apple Inc.

src/codes/appclip/mod.rs

@@ -20,8 +20,9 @@
 //! foreground or a derived third color. See `doc/SPEC.md` of [rs/appclipcode] for the
 //! full write-up.
 //!
-//! Encoding and structural decoding (bits → URL) are implemented; camera detection of
-//! the printed circle is not yet wired into the live pipeline.
+//! Encoding, structural decoding (bits → URL) and camera detection ([`scan`], a
+//! grayscale ring detector + sampler over a [`crate::image::GrayFrame`]) are all
+//! implemented; [`crate::pipeline::scan_2d`] runs the detector automatically.
 //!
 //! [rs/appclipcode]: https://github.com/rs/appclipcode
 //!
@@ -45,11 +46,13 @@
 mod codec;
 mod gf;
 mod huffman;
+mod scan;
 mod svg;
 mod tables;
 mod url;
 
 pub use codec::{decode_payload, encode_payload};
+pub use scan::scan;
 pub use svg::{Color, LogoKind, Palette, template_palette, third_color};
 pub use url::{compress_url, decompress_url};