Read WebP metadata past the scan window and surface C2PA reader failures

Three gaps found while measuring the record path against the file path, each one
a signal the library could not see:

WebP stores `XMP ` after the pixels, so on any WebP above the scan window a fixed
read stops short of the label. `_riff_late_metadata` steps over the coded image to
reach it, the RIFF analogue of the existing PNG and ISOBMFF readers. Three corpus
files hid an IPTC "Made with AI" tag and a C2PA `trainedAlgorithmicMedia` there.
The decoder-backed fallback now covers only what it is actually for -- metadata the
raw bytes do not spell, such as a compressed PNG `zTXt` packet.

A C2PA reader failure returned the same `None` as a file with no manifest, so a
verdict could fall back to the raw byte scan with no trace anywhere. Failures now
log at warning and only genuine ones do: a file without credentials never reaches
that branch, and an unsupported container is demoted to debug through the reader's
own `C2paError.NotSupported`. The first corpus run with it found a truncated PNG.

`scan_dataset.py` never registered the pillow-heif opener it declares as a
dependency, so every HEIC was scanned as unreadable -- no EXIF, and a pixel layer
that was 397 of 406 features NaN instead of 136.

`_riff_late_metadata` caps its total like `isobmff.scan_c2pa_region` does. Clamping
each chunk to the bytes remaining is not enough on its own: one chunk can declare a
length spanning most of the file, and this runs on the memoized verdict path over
images from arbitrary sources.

Also lands `identify_metadata_record` and `ProvenanceReport.to_dict()`, the
one-call entry point and the versioned JSON contract for the record path.

Record-vs-file equality holds over 3,478 corpus images, and the eight files these
fixes recovered still report AI.

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

scripts/detection_timing.py

@@ -39,7 +39,6 @@ DATA SAFETY
   Read-only over a local dataset. Writes only to the given output prefix, which
   belongs outside the repository.
 
-    uv run python scripts/detection_timing.py data/spaces/originals .local-eval/timing/run
     uv run python scripts/detection_timing.py <dataset> <prefix> --limit 200
 """
 

scripts/scan_dataset.py

@@ -72,6 +72,18 @@ from typing import Any
 from PIL import Image
 from PIL.IptcImagePlugin import getiptcinfo
 
+# Pillow cannot open HEIC/HEIF without this opener, and it does not auto-register.
+# Skipping it does not fail loudly: HEIC records lose EXIF and most pixel features
+# while the scan continues as if the container were merely unreadable. Deliberately a
+# copy of `image_io._register_heif` rather than an import: this script keeps the
+# minimal dependency set its docstring advertises and never imports the package. The
+# suppression is as wide as the original's, so a broken libheif degrades the scan
+# instead of killing it at import.
+with contextlib.suppress(Exception):
+    import pillow_heif
+
+    pillow_heif.register_heif_opener()
+
 SUPPORTED = {
     ".png",
     ".jpg",