fix: metadata-strip parity, input robustness, and detection/clash coverage

Bug fixes (each with a regression test):
- metadata strip parity across every marker placement: IPTC digitalSourceType
  in XMP, the Samsung post-EOI trailer, the China TC260 AIGC block in EXIF
  UserComment, a bare AIGC block in a non-standard APP segment, and the ISOBMFF
  EXIF path (AIGC + xAI) are all now stripped -- anything a scanner flags, the
  strip reaches
- Samsung genAIType detected when its trailer sits past the 512 KB scan window
  (file-tail read on large photos)
- crashes on edge inputs: Gemini detector on images with a short side < 16px,
  footprint_mask on a zero-size ndarray, the humanizer on chromatic_shift >=
  width, and the CLI on unreadable/corrupt/empty input (clean error, not a
  traceback)
- WebP written losslessly (cv2 quality 101), not lossy at 100
- the IPTC digitalSourceType algorithmicMedia (procedural, not trained on
  sampled data) is no longer flagged as AI-generated, so clean procedural
  content is not scrubbed
- c2pa source-type: compositeWithTrainedAlgorithmicMedia is checked before the
  bare algorithmicMedia token, so an AI-enhanced composite is not misclassified

Detection:
- integrity-clash coverage now normalizes ByteDance / Canva / ElevenLabs /
  Black Forest Labs, so a transplanted manifest next to an independent
  conflicting stamp is caught; the generic China TC260 AIGC label is attributed
  to a co-present TC260 vendor, so a legit Doubao image (its own C2PA + TC260
  label) does not clash (corpus-validated: 0 new clashes on 5000 carriers)

CLI:
- batch exits non-zero (with a warning) when any image errors or a GPU-missing
  SynthID scrub is skipped, and copies the input through so the output dir stays
  complete -- it used to always exit 0 and could silently drop files

Perf:
- GeminiEngine reused as a process-wide singleton with a precomputed template
  ladder: -24% on the identify sparkle path, detection byte-identical

Internal: one shared _ai_exif_targets rule set feeds both EXIF scrubbers so
their coverage cannot drift; docs synced; maintain.sh hardened so the uv-secure
internal teardown crash no longer aborts the gate (still fails on a real finding).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/known-limitations.md

@@ -74,7 +74,7 @@ Metadata detection for AVIF/HEIF/JPEG-XL relies on a binary scan for `C2PA_UUID`
 
 **Meta-box XMP now handled (`isobmff.blank_ai_xmp_packets`, v0.6.9):** an AI-label XMP packet stored as a meta-box `mime` item (AVIF/HEIF) is blanked in place (overwritten with spaces of the same length, so `iloc` offsets and the coded image stay valid).
 
-**`Exif` item inside the `meta` box (AVIF/HEIF), now handled in place (2026-06-19):** an AI-generator token in an EXIF item (its TIFF bytes live in `mdat`/`idat`) is blanked by `isobmff.blank_ai_exif_tokens` — it finds EXIF TIFF blocks by their II/MM byte-order header, validates each with **piexif** (a coincidental II/MM run in pixel data won't parse as a TIFF IFD, so it is ignored), and overwrites any `Software`/`Make`/`Artist`/`ImageDescription` value carrying an `AI_GENERATOR_TOKENS` token with spaces of the **same length**. Same-length means every box size and `iloc` offset stays valid and the coded image is untouched — so it avoids the full `iinf`/`iloc` surgery (offset rewrite) that exiftool would need (exiftool is a non-installed binary dep, deliberately not used). It scrubs only the AI-token value; camera/editor EXIF is preserved. Wired into `remove_ai_metadata`'s ISOBMFF path after `blank_ai_xmp_packets`. Limitation: covers the AI-generator-token case (the realistic one); a future xAI-signature-in-meta-box-EXIF (Grok is JPEG-only today) is not separately handled. **Still NOT built:** Resemble PerTh audio detection (no presence/confidence flag exists).
+**`Exif` item inside the `meta` box (AVIF/HEIF), now handled in place (2026-06-19):** an AI-generator token in an EXIF item (its TIFF bytes live in `mdat`/`idat`) is blanked by `isobmff.blank_ai_exif_tokens` — it finds EXIF TIFF blocks by their II/MM byte-order header, validates each with **piexif** (a coincidental II/MM run in pixel data won't parse as a TIFF IFD, so it is ignored), and overwrites any `Software`/`Make`/`Artist`/`ImageDescription` value carrying an `AI_GENERATOR_TOKENS` token with spaces of the **same length**. Same-length means every box size and `iloc` offset stays valid and the coded image is untouched — so it avoids the full `iinf`/`iloc` surgery (offset rewrite) that exiftool would need (exiftool is a non-installed binary dep, deliberately not used). It scrubs only the AI value; camera/editor EXIF is preserved. Wired into `remove_ai_metadata`'s ISOBMFF path after `blank_ai_xmp_packets`. Because the ISOBMFF branch never runs the JPEG `_scrub_ai_exif`, this is the ONLY EXIF scrubber on that path and must stay in PARITY with it: it now also blanks the China TC260 `{"AIGC":{...}}` block in `ImageDescription`/`UserComment` (via `_is_aigc_exif_value` — Doubao producer + Tencent service-provider schemas) and the xAI/Grok `Signature:` + UUID-`Artist` pair, not just `AI_GENERATOR_TOKENS` in `Software`/`Make`/`Artist`/`ImageDescription` (regression `test_noai.py::TestISOBMFF::{test_blank_aigc_block_in_exif, test_blank_xai_signature_pair_in_exif}`). **Still NOT built:** Resemble PerTh audio detection (no presence/confidence flag exists).
 
 **Audio watermark DETECTION (Resemble PerTh) was evaluated and NOT built (2026-05-26):** `resemble-perth`'s `PerthImplicitWatermarker.get_watermark()` returns a raw bit-array with **no presence/confidence flag** (clean audio decodes to arbitrary bits too), so reliably distinguishing watermarked-from-clean needs either Resemble's fixed payload or a confidence API -- neither is public, and there's no real Resemble sample to calibrate against. Same wall-class as the SynthID pixel detector: the decode exists, reliable presence-detection does not. (perth's top-level `PerthImplicitWatermarker` is also gated to None unless `librosa` is importable.)