Fail closed on unreadable inputs, empty batches and failed strips

The metadata scanners read raw bytes, so random bytes, an empty file or a
truncated PNG or MP4 printed the same "No AI metadata found" or "AI metadata
stripped" a real file gets, with exit 0. `metadata.unreadable_reason` is now
the one readability probe: `identify`, `metadata` and every `video` command
exit 1 on such input, and `batch` and `video batch` fail its row.
`strip_and_verify` refuses it before writing anything.

A `batch` or `video batch` directory with no supported file exits 2 and
creates no output directory. `visible` and `erase` strip and verify on a
staged sibling and publish nothing when a marker survives, as the library
`remove_visible` now does; the diffusion writer raises on a surviving
marker. An invisible engine that cannot start exits 1 without a traceback,
and the MCP server reports a child that could not start.

NotebookLM and Gemini slide-deck templates are recorded as not planned.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/assets/visible-mark-candidates/README.md

@@ -22,7 +22,7 @@ from the provider's canonical name, the provider is named separately.
 | --- | --- | --- | --- |
 | `samsung_en_alpha.png` | Samsung Galaxy AI label, English locale ("AI-generated content", bottom-left) | 5 corpus files | POS 0.11-0.30 vs NEG max 0.40 (binary); tophat/gray tried, no separation. The registered Italian engine scores 0.18-0.31 on the same files -- same layout class, wrong glyph template. |
 | `gemini_text_alpha.png` | "Generated with Gemini" text label (bottom-right; the registered gemini mark is the sparkle icon) | 3 corpus files | POS 0.07-0.22 vs NEG max 0.32; coverage gate finds the blob (0.29-0.35) but the Arial silhouette misses Google's letterforms. |
-| `notebooklm_alpha.png` | NotebookLM wordmark (bottom-right) | 12 corpus files | locate geometry not yet fitted; POS max 0.12. |
+| `notebooklm_alpha.png` | NotebookLM wordmark (bottom-right; the product is now Gemini Notebook) | 12 corpus files | parked: a template is not planned (decided 2026-10-18). Last measured POS max 0.12, locate geometry never fitted. |
 | `mindvideo_alpha.png` | MindVideo.AI (top-right) | 11 corpus files | POS 0.29-0.32 vs NEG max 0.30 -- borderline overlap, not shippable. |
 | `higgsfield_alpha.png` | `HIGGSFIELD AI` wordmark (Higgsfield AI, bottom-right; the boxed `AI` variant shares the cohort) | 5 wordmark files (16 in the boxed-AI OCR cluster) | POS max 0.26 vs NEG max 0.22 -- no separation; the mark may be two-part (wordmark + boxed AI) and needs a composed template. |
 | `jianying_alpha.png` | `剪映AI` wordmark (Jianying / 剪映, the China product in the CapCut family; bottom-right) | 2 corpus files | POS 0.29 vs NEG max 0.35. |