feat(examples): add open-slide compatibility proof

[rare]

Summary

• add a pinned open-slide 1.17.1 multi-file compatibility fixture and live reader evidence
• extend the shared reader bridge for opaque-origin storage, nested artifact routing, and window keyboard navigation
• add fail-closed tests, build validation, and a known-advisory CI gate

Verification

• Node 22: npm run build (10 files, 1,065,017 bytes)
• node --test examples/inject-slidesfly-reader-bridge.test.mjs examples/message-actions.test.mjs (12/12)
• npm run audit:known (5 known advisory URLs unchanged; no critical advisory)
• local root and simulated /proof/1/ browser paths rendered with 0 errors and 0 warnings
• live reader rendered and Arrow Right advanced page 1 to page 2
• versioned index byte-matched SHA-256 e6d6bf27260ce8db448964392b92c6a63ddafe4bb736677b64f9919ef6a777eb

Live proof

https://slidesfly.xyz/d/G38P7MGbJ2eMyFMmITVWc2

Boundaries

This is an independent compatibility fixture, not an upstream endorsement. The standard open-slide build is multi-file and therefore uses an eligible authenticated Slidesfly plan.