C14.10 r1: no line break in an image's alt text; names that are no text are refused before copying

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tasks/reviews/C14.10-r1.md

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+- apps/ely-mark/src/attach.rs:138 — Literal line breaks in the filename are copied into image alt text. On Linux, dropping `a\n\nb.png` copies the file successfully but inserts `![a\n\nb](assets/a%0A%0Ab.png)`. markdown-rs parses this as two ordinary paragraphs, with no image. Encode CR/LF safely in the alt text and add a regression that checks the generated Markdown parses as an image. Reproduced with the original `link`/`escaped` functions and markdown-rs; output: `.work/C14.10-r1/linux-probe.log`.
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+- apps/ely-mark/src/attach.rs:135 — `to_string_lossy()` silently changes an attachment's actual filename when building its address. On WSL Linux, importing the valid filename `b"pic-\xFF.png"` preserves those bytes on disk, but the generated link is `assets/pic-%EF%BF%BD.png`, which names a different, nonexistent file. No failure is reported. Remove the lossy conversion from address generation; either support the actual filename end to end or explicitly reject unrepresentable names before copying. Add a regression using a real non-UTF-8 filename. Reproduced with `ely_files::import_attachment` and the original link functions; output: `.work/C14.10-r1/linux-probe.log`.