docs: add SN66 public R2 data feed URL and path reference

[ProjectNobi]

Adds a ## SN66 Public Data Feed section to the README documenting the public base URL and key path patterns.

Covers:

• Public read base: https://pub-0821b4e0d60149b79bad17376722bc75.r2.dev/
• Key paths: sn66/dashboard.json, sn66/duels/index.json, sn66/duels/{id}/duel.json, sn66/duels/{id}/rounds/...
• Quick curl example
• updated_at as freshness signal
• Pointer to src/r2.py for write-side key construction
• Caveat about pub-… ID potentially changing if ops rebinds

Context: unarbos/sn66-validator#1

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