feat(metadata): SynthID-source detection, C2PA parser consolidation, corpus + tests

Detect SynthID-bearing images via their C2PA companion: a manifest signed by a
SynthID-using vendor (Google/OpenAI) on AI-generated content implies an
invisible SynthID pixel watermark. Verified end-to-end against the vendor
oracles (openai.com/verify, Gemini "Verify with SynthID").

- metadata: synthid_source() + synthid_watermark verdict in get_ai_metadata,
  surfaced as a `metadata --check` callout. Format-agnostic (PNG caBX parser +
  JPEG/WebP/AVIF/HEIF/JXL binary scan).
- constants: SYNTHID_C2PA_ISSUERS {Google, OpenAI}; +opened/placed actions.
- c2pa: single CBOR-aware parser (_cbor_text_after) replaces glitchy regex
  (fixes fGPT-4o claim_generator); removed duplicate _scan_png_c2pa_chunk from
  metadata; shared synthid_verdict / synthid_vendors_in helpers.
- corpus: scripts/synthid_corpus.py ingest tool + data/synthid_corpus/
  (manifest tracked, images gitignored) for a labeled reference set.
- tests: +38 across C2PA parser internals, extract/inject round-trip, ISOBMFF
  container stripping, all IPTC AI markers, and invisible watermark strength
  tiers (SynthID/StableSignature/TreeRing/StegaStamp/RingID/RivaGAN/...).

Pixel-level SynthID detection remains out of reach locally (Google's decoder is
proprietary); a from-scratch spectral pilot confirmed it does not separate real
content. See CLAUDE.md for the full evaluation.

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

tests/test_platform.py

@@ -87,6 +87,24 @@ class TestModelProfiles:
     def test_recommended_strength_medium(self):
         assert get_recommended_strength("unknown_type") == 0.35
 
+    @pytest.mark.parametrize("wm_type", ["stegastamp", "stegasamp", "treering", "ringid"])
+    def test_high_perturbation_watermark_types(self, wm_type):
+        """Robust spatial watermarks need aggressive (0.7) regeneration."""
+        assert get_recommended_strength(wm_type) == 0.7
+
+    @pytest.mark.parametrize("wm_type", ["stablesignature", "dwtectsvd", "rivagan", "ssl", "hidden"])
+    def test_low_perturbation_watermark_types(self, wm_type):
+        """Fragile frequency/latent watermarks break at low (0.04) strength."""
+        assert get_recommended_strength(wm_type) == 0.04
+
+    def test_strength_match_is_case_insensitive(self):
+        assert get_recommended_strength("TreeRing") == 0.7
+        assert get_recommended_strength("StableSignature") == 0.04
+
+    def test_strength_matches_substring_in_descriptive_name(self):
+        # e.g. a CLI passing "treering_v2" or "synthid-stegastamp" still maps.
+        assert get_recommended_strength("treering_v2") == 0.7
+
 
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