[Rewrite P2] NIP currency: implement audit-identified gaps in Rust #870

Parent: #860
Milestone: Phase 2: Protocol core
Host: chappie · Expected cost: $450

Implement the NIP gaps Phase 0's audit finds, in the Rust core/relay — protocol currency is judged at the capture date, and the audit (#861) produces the authoritative list.

• Split from [Rewrite P0] Protocol currency audit: NIP registry vs nostream at capture #861's disposition table: each must-have gap becomes a row here (or its own issue if it's large)
• Each implemented NIP lands with its spec's test vectors + the differential harness from [Rewrite P2] nostream-relay: wire handlers, strategies, rate limits #865 covering it
• Anything deferred carries the reason in docs/rewrite/nip-parity.md

Acceptance: supportedNips in the Rust relay matches #861's audited must-have list; every advertised NIP has implementation + vectors.