Add reproducible WebMCP proof pack

[TueJon]

TUE-377 proof deliverable.

What changed:

• adds a sanitized release-notes fixture and before/after Manifest v3 evidence;
• records a 63.08-second uncut 1280x720 source run plus a 480p H.264 fast-start derivative and SHA-256 sums;
• uses headed system Chrome 150 with native document.modelContext getTools/executeTool;
• demonstrates inventory, a real approval click, bounded integration, schema/annotation checks, valid execution with a 4-to-2 UI delta, bounded invalid input, cleanup, and audit;
• documents exact reproduction commands.

Verification:

• npm run check
• npm run proof:verify
• sha256sum -c proof/artifacts/SHA256SUMS
• jq empty proof/manifest.before.json proof/manifest.after.json
• git diff --check

No deployment or external publication was performed. WebMCP is described as an origin-trial API.