feat: add EPUB and KEPUB derivatives

[szmyty]

Summary

• add first-class epub and kepub output types and a visible two-stage EPUB -> KEPUB graph path
• generate reflowable EPUB 3 with Pandoc and convert KEPUB with Kepubify through the bounded local process service
• add versioned renderflow.ebook-evidence/v1 structural inspection for package metadata, XHTML, ordered spine, navigation, page-list, layout, accessibility, source digest, and KEPUB markers
• integrate optional EPUBCheck JSON evidence and report missing validators/providers honestly
• register Kepubify and EPUBCheck in the tool and adapter catalogs with adopt/defer rationale
• preserve conventional .kepub.epub final naming and reject unsupported bundled fixed-layout generation
• document provider-neutral capability boundaries so retailer eligibility remains in downstream packs such as Lulu

Verification

• cargo check --workspace --all-targets
• targeted Clippy for workspace libraries/binaries (existing unrelated broad Clippy findings excluded)
• real Pandoc EPUB generation and native structural inspection smoke
• KEPUB two-wave graph-plan smoke
• missing Kepubify preflight/failure-evidence smoke
• adapter catalog and capability-report smokes
• JSON schema parsing and git diff --check

Broad tests were intentionally deferred for the current sprint.

Closes #344

[devactivity-app]

Pull Request Summary by devActivity

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