[OC-011] Implement long-term and derived identities

[tcballard]

Gate: G1 — Nostr-only product

Planning source: validated implementation plan PR #1

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• Depends on: OC-004, OC-010.
• Deliverable: Noise static, Ed25519 signing, separate Nostr device seed, peer fingerprint/ID, per-geohash and bridge|cell secp256k1 derivation.
• Acceptance: all identity fixtures pass, invalid secp scalars retry exactly, fallback derivation is tested, and regeneration happens only after explicit identity absence.
• Excludes: favorites and QR.

Maintainer reconciliation — 2026-09-04

Next closure evidence/work: Close candidate: attach identity fixtures, invalid-scalar retry/fallback, bridge-domain separation and restart/no-regeneration tests before closing.

Baseline reviewed: main 87c3bcb7d65e3e2ab8ae317694b751f8efe9a8ed. Implementation presence is not a live conformance claim.

Verified closure — 2026-09-04

Checked every deliverable and acceptance criterion on main 87c3bcb7d65e3e2ab8ae317694b751f8efe9a8ed.

• Identity implementation: independent secret roots, Noise static key, Ed25519 signing, fingerprint/peer ID, geohash derivation and separate bridge|cell domain.
• Identity fixture tests: pinned Swift private/public/npub outputs and signing/key-role checks.
• Inline unit tests prove exact invalid-scalar retry order, fallback only after ten invalid candidates, and bridge domain separation.
• Persistence implementation generates only for RecordNotFound; decoding/authentication/I/O errors propagate without replacement.
• Persistence regression proves restart-stable identity and failure on tampered ciphertext.

CI job 100951293375 explicitly records these tests passing, plus identity_outbox_and_commands_survive_restart. This is the successful pre-merge CI for #222's merged tree. No fresh local Rust run is claimed.

Closed as implemented within OC-011 scope. Secret Service host qualification, favorites and QR remain separate issues.