[FEAT]: Add EPUB export diagnostics warnings

[anantham]

Summary

Add structured export warnings for missing translations/image cache misses and package validation logs without changing EPUB output. Warnings are surfaced in progress messages and telemetry to improve diagnostics without altering EPUB contents.

Root Cause (if bug fix)

N/A (observability improvement)

Changes

• store/slices/exportSlice.ts: record structured warnings and surface warning summary/telemetry.
• services/epubService/packagers/epubPackager.ts: add package validation warnings.
• tests/services/epubPackager.diagnostics.test.ts: verify warning emission.
• docs/WORKLOG.md: session log updates.

Testing

• npx vitest run tests/services/epubPackager.diagnostics.test.ts

Review Checklist

• No direct commits to main
• Follows commit format
• Ready for Codex review
• No unrelated changes mixed in

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Project	Deployment	Review	Updated (UTC)
lexicon-forge	Ready	Preview, Comment	Jan 8, 2026 8:18pm

[anantham]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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