Track the labeled SynthID corpus; complete metadata-source test coverage

Corpus images were gitignored (local-only). The negatives were reviewed and
cleared for publishing, so the labeled set is now committed (regular git, 65 MB
across 25 files) -- making the removal regression set reproducible and CI-able.

Corpus:
- Track data/synthid_corpus/images/ (pos 9, neg 15, cleaned 1); keep only the
  synthetic refs/ calibration fills gitignored.
- Reconcile manifest.csv to the on-disk files: 117 -> 25 rows (92 dangling rows
  for removed images pruned; dedup left one cleaned output, f6dd47a5).
- Rewrite the corpus README layout/policy (images committed; review every image
  for private content before adding -- public repo, permanent history).

Test fixtures:
- Remove data/samples/not-ai-1/2/3 (personal iPhone photos, incl. GPS EXIF).
- Add the clean_photo conftest fixture serving a verified-negative image from
  the corpus neg/ set; repoint the three "non-AI / clean photo" tests onto it
  (skips if the corpus is absent).

Metadata-source coverage (close the last sub-variant gaps):
- c2pa digitalSourceType: algorithmicMedia (procedural, not flagged AI) and
  compositeWithTrainedAlgorithmicMedia (AI + SynthID proxy).
- exif_generator: EXIF Artist and ImageDescription fields (Software/Make/XMP
  CreatorTool were already covered).

All 8 metadata-source kinds are now tested at both the unit and identify()
level. 313 tests pass. CLAUDE.md updated (corpus tracked, clean_photo fixture).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_identify.py

@@ -156,15 +156,15 @@ class TestIdentifyRealSamples:
         assert r.is_ai_generated is True
         assert any("IPTC" in w for w in r.watermarks)
 
-    def test_clean_photo_is_unknown_not_clean(self):
-        r = identify(SAMPLES_DIR / "not-ai-1.jpeg", check_visible=False)
+    def test_clean_photo_is_unknown_not_clean(self, clean_photo: Path):
+        r = identify(clean_photo, check_visible=False)
         assert r.is_ai_generated is None  # never asserted False
         assert r.platform is None
         assert r.confidence == "none"
         assert r.watermarks == []
 
-    def test_strip_caveat_always_present(self):
-        r = identify(SAMPLES_DIR / "not-ai-1.jpeg", check_visible=False)
+    def test_strip_caveat_always_present(self, clean_photo: Path):
+        r = identify(clean_photo, check_visible=False)
         assert any("not proof" in c for c in r.caveats)
 
     def test_returns_report_dataclass(self):