Implement bounded deterministic fixed-layout EPUB generation (#417)

[szmyty]

Closes #417. Parent #414; builds on #415.

Implementation

• Adds the exact native ebook.generate.epub.fixed-layout route for a homogeneous, explicitly ordered local PNG or JPEG collection. Planning requires frozen IDs, digests, page geometry, a first-member cover, reviewed per-page descriptions, publication metadata, and explicit page/input/output byte bounds.
• Writes byte-identical EPUB packages from clean builds: stored first mimetype, fixed member order and timestamps, EPUB 3 OPF/XHTML, fixed viewports, spine, cover image, navigation, page list, LTR/RTL progression, and declared accessibility metadata. Original image bytes and sources remain unchanged.
• Rechecks source structure, dimensions, digest, and package bounds while writing; imports only complete output. Typed refusal and cancellation evidence covers unsupported media, unsafe/malformed SVG refusal, stale sources, missing metadata, and output limits.
• Updates the tool catalog, v2 schema/reference, CLI coverage, and user/roadmap docs. Pandoc's reflow route and fixed-layout KEPUB claims remain separate. Broad ebook capabilities advertisement and independent EPUB 3.3 conformance stay with [Checkpoint 414.3] Validate fixed-layout EPUB and publish capability truth #418.

Local validation

• cargo test --workspace --locked --offline --quiet passed (1,218 tests; 12 existing ignored). The final additive focused cases then passed: fixed_layout_epub 10/10 and fixed_layout_epub_cli 1/1.
• cargo clippy --workspace --all-targets --locked --offline -- -D warnings passed.
• cargo fmt --all --check, schema/reference regeneration, git diff --check, and strict MkDocs build passed.
• Synthetic artwork only; no real media, network publication, retailer submission, or hosted CI run was invoked.

The Git Data published tree was fetched back and matches the validated local tree d6acf84b2aaccf926dc1c3e0e8e3a10ac1562575.

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