Decide the SynthID proxy in the verdict, where both extractors meet

A full-corpus audit of the record path against the file path found 75 of 48,905
images disagreeing, and 74 were one gap: the SynthID byte scan for containers whose
manifest no parser reaches lived in `get_ai_metadata`, an extractor the record path
does not run. The record silently reported no SynthID for images `identify` flagged.

Moving the scan into `identify_from_evidence` fixes it by construction rather than by
copying the rule into a second extractor -- the same shape `soft_binding` already
uses. Its byte checks mirror `metadata.synthid_source` literally instead of reusing
the broader `has_c2pa` / `c2pa_source_kind` derived above, so the file path's answers
do not move: verdicts over a 4,000-image sample are byte-identical.

`scripts/record_parity_audit.py` is the audit itself, now repeatable. It walks a
dataset, judges every image through both seams with the record round-tripped through
JSON, and reports disagreements by field and by signal. The rule in
`.claude/rules/development.md` says to re-run both sides of this seam after changing
either; this is what to run.

Both timing and audit scripts now put the package's OWN `src` on the path. From a
worktree an editable install resolves to the main checkout, so the audit imported a
different tree than the one under test -- the failure the same rules file warns about,
reproduced within an hour of writing it down.

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

scripts/detection_timing.py

@@ -55,7 +55,10 @@ from typing import TYPE_CHECKING, Any
 if TYPE_CHECKING:
     from collections.abc import Callable, Iterator
 
-sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
+# The package's OWN tree, not the repository root: from a worktree, an editable
+# install resolves `remove_ai_watermarks` to the MAIN checkout, so a script measuring
+# this tree would silently import a different one.
+sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "src"))
 
 from remove_ai_watermarks import identify as identify_mod
 from remove_ai_watermarks import metadata as metadata_mod