Add credit attestation ledger

[evilfps]

/claim #15

Adds a contributor credit attestation ledger for community reputation workflows.

Covers:

• CRediT role validation and evidence checks
• independent attestation strength
• duplicate credit claim detection
• open dispute and attestor conflict routing
• profile summaries, reputation deltas, signed events, and manifest digest
• sample data, tests, requirement map, SVG, GIF, and WebM demo

Demo:

• credit-attestation-ledger/docs/demo.webm
• credit-attestation-ledger/docs/demo.gif

Tested:

• cd credit-attestation-ledger && npm run check
• cd credit-attestation-ledger && npm test
• cd credit-attestation-ledger && npm run demo
• rsvg-convert credit-attestation-ledger/docs/demo.svg -o /tmp/credit-demo.png
• demo.gif opened with Pillow: GIF, 3 frames, 960x540
• gst-discoverer-1.0 credit-attestation-ledger/docs/demo.webm: VP8 WebM, 3s, 960x540
• git diff --check
• credential pattern scan, no hits

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