Export fidelity regression suite: PPTX/PDF/PNG round-trip goldens

[tph-kds]

Context: deckforge-export adds/repairs export capabilities; VALIDATION_REPORT.md records 6 export contracts.

Gap: export fidelity is validated manually or implicitly; regressions in fidelity (layout drift, missing blocks, broken links) can ship silently.

Acceptance:

• Golden-reference comparisons for PPTX, PDF, and PNG exports of a representative deck.
• Fidelity thresholds (layout drift, block coverage, text integrity) defined per export contract.
• CI job that regenerates and compares goldens; failures block release.