Implement bounded ordered print-interior PDF generation (#416)

[szmyty]

Summary

• Add the exact publication.generate.pdf.interior route from an immutable, ordered PNG or JPEG collection to one print-interior PDF. Require explicit uniform trim/bleed geometry, a pinned and observed img2pdf 0.6.3 provider, direct bounded argv, and a required interior target.
• Preflight source image bytes and color/geometry constraints; recheck source identity before execution. Independently inspect the produced PDF's page count, ordered embedded image-stream hashes, full-page placement, media/bleed/trim boxes, rotation, color space, and output digest before materialization. Persist ordered source lineage and typed provider/inspection failure evidence.
• Keep cancellation, timeout, nonzero exit, missing/stale/invalid output, and unsupported inputs from publishing a PDF. Document the exact capability, schemas, catalogs, and roadmap handoff.

Local validation

• cargo test --workspace --locked --quiet with real img2pdf 0.6.3: 1,210 passed, 11 ignored, 0 failed. Synthetic route tests include PNG and JPEG clean-build determinism, a 44-page recipe, source/refusal cases, provider failures, and in-flight cancellation.
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo fmt --all --check
• Runtime renderflow spec schema matches the checked-in JSON Schema; strict MkDocs build passed.
• Public CLI build --dry-run and build produced one validated PDF from synthetic pages.
• Published Git tree 6b080da6c2dee3c46ac504334f3fa034366ee2a6 matches the locally validated tree exactly.

No real publication pages were processed. Print/retailer acceptance and calibrated color reproduction are outside this bounded route.

Closes #416

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