Add mergeable NIP-45 HLL counts

[erskingardner]

Summary

• add NIP-45 fixed-precision HyperLogLog sketches to eligible COUNT responses so clients can merge distinct-author registers across relays
• implement the complete deterministic offset rule for event/pubkey hex, address pubkeys, and SHA-256-hashed arbitrary target strings
• emit sketches only for canonical one-filter/one-tag/one-target requests and omit them from multi-target or limited responses where a sketch would be ambiguous or incomplete
• cover bit-rank vectors, offset forms, exact-count versus distinct-author behavior, empty sketches, ambiguous filters, and response encoding

Validation

• cargo fmt --all -- --check
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings

Stack

• depends on Implement restart-safe NIP-62 vanish requests #4
• Implement restart-safe NIP-62 vanish requests #4 depends on Bound query results and stress historical scans #3, which depends on Make NIP-59 privacy safe by default #2

References:

• https://github.com/nostr-protocol/nips/blob/master/45.md
• strfry proposal: Add HyperLogLog support for COUNT (NIP-45) hoytech/strfry#181