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/conftest.py

@@ -2,10 +2,7 @@
 
 from __future__ import annotations
 
-from typing import TYPE_CHECKING
-
-if TYPE_CHECKING:
-    from pathlib import Path
+from pathlib import Path
 
 import cv2
 import numpy as np
@@ -13,6 +10,22 @@ import pytest
 from PIL import Image
 from PIL.PngImagePlugin import PngInfo
 
+CORPUS_NEG_DIR = Path(__file__).resolve().parent.parent / "data" / "synthid_corpus" / "images" / "neg"
+
+
+@pytest.fixture
+def clean_photo() -> Path:
+    """A verified-negative real photo from the corpus neg/ set.
+
+    Used by the "non-AI image" assertions (no SynthID, verdict unknown). These
+    are real photos with no AI provenance, the ground truth for "must not false-
+    positive". Skips if the corpus is not checked out.
+    """
+    files = sorted(CORPUS_NEG_DIR.glob("*")) if CORPUS_NEG_DIR.exists() else []
+    if not files:
+        pytest.skip("no corpus neg/ images present")
+    return files[0]
+
 
 @pytest.fixture
 def tmp_image_path(tmp_path: Path) -> Path:

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):

tests/test_metadata.py

@@ -224,8 +224,8 @@ class TestSynthIDSource:
         assert synthid_source(SAMPLES_DIR / "firefly-1.png") is None
         assert "synthid_watermark" not in get_ai_metadata(SAMPLES_DIR / "firefly-1.png")
 
-    def test_non_ai_image_is_not_synthid_source(self):
-        assert synthid_source(SAMPLES_DIR / "not-ai-1.jpeg") is None
+    def test_non_ai_image_is_not_synthid_source(self, clean_photo: Path):
+        assert synthid_source(clean_photo) is None
 
 
 class TestSynthIDSourceNonPng:
@@ -377,6 +377,24 @@ class TestExifGenerator:
         Image.new("RGB", (64, 64)).save(path, exif=exif)
         assert exif_generator(path) is None
 
+    def test_artist_tag_ai_tool_detected(self, tmp_path: Path):
+        # exif_generator also reads the EXIF Artist field for an AI token.
+        exif = piexif.dump({"0th": {piexif.ImageIFD.Artist: b"Midjourney"}, "Exif": {}, "GPS": {}, "1st": {}})
+        path = tmp_path / "artist.jpg"
+        Image.new("RGB", (64, 64)).save(path, exif=exif)
+        assert exif_generator(path) == "Midjourney"
+
+    def test_imagedescription_tag_ai_tool_detected(self, tmp_path: Path):
+        # ...and the EXIF ImageDescription field.
+        exif = piexif.dump(
+            {"0th": {piexif.ImageIFD.ImageDescription: b"Made with Stable Diffusion"}, "Exif": {}, "GPS": {}, "1st": {}}
+        )
+        path = tmp_path / "desc.jpg"
+        Image.new("RGB", (64, 64)).save(path, exif=exif)
+        result = exif_generator(path)
+        assert result is not None
+        assert "Stable Diffusion" in result
+
     def test_xmp_creatortool_scan_covers_unopenable(self, tmp_path: Path):
         # PIL can't open this fake HEIF; the raw XMP CreatorTool scan still works.
         path = tmp_path / "fake.heic"

tests/test_noai.py

@@ -259,6 +259,29 @@ class TestParseChunkGuards:
         assert "OpenAI" in info["issuer"]  # issuer byte-search still robust
 
 
+class TestC2PADigitalSourceType:
+    """The three IPTC digitalSourceType variants drive the AI verdict.
+
+    Only *trained* and *composite-with-trained* mean AI-generated (and so imply
+    a SynthID proxy for a SynthID vendor); plain ``algorithmicMedia`` is
+    procedural (not trained) and must NOT be flagged as AI.
+    """
+
+    def test_plain_algorithmic_media_not_flagged_ai(self):
+        chunk = b"...name" + bytes([0x69]) + b"some-tool" + b" OpenAI algorithmicMedia"
+        info: dict = {}
+        _parse_c2pa_chunk(chunk, info)
+        assert info["source_type"] == "algorithmicMedia"
+        assert "synthid_watermark" not in info  # procedural, not AI-generated
+
+    def test_composite_with_trained_is_ai_and_synthid(self):
+        chunk = b"...name" + bytes([0x69]) + b"some-tool" + b" OpenAI compositeWithTrainedAlgorithmicMedia"
+        info: dict = {}
+        _parse_c2pa_chunk(chunk, info)
+        assert "compositeWithTrainedAlgorithmicMedia" in info["source_type"]
+        assert "synthid_watermark" in info  # AI-enhanced + OpenAI issuer
+
+
 # ── ISOBMFF (AVIF / HEIF / JPEG-XL container stripping) ──────────────
 
 FTYP = b"\x00\x00\x00\x18ftypavif\x00\x00\x00\x00avifmif1"  # 24-byte ftyp box