Third slice: bridge Block 1's root seed to the Block 2 transport foundation.
- SocialService: derives the secp256k1 Nostr identity from the SAME root seed
the recovery QR backs up (no extra backup), exposes npub/pubkey, and opens a
SocialSession = one shared connection carrying all three transports.
- injector.dart: registers SocialService in get_it. Local-first — derivation is
cheap + offline; no relay is contacted at startup. Replaces the old
"nodeId slice of the seed is the social pubkey" placeholder; keeps the CRDT
author id distinct (unifying it would rewrite authorship — a later decision).
- Test: deterministic identity from the seed hex, matches commons_core, rejects
malformed hex. 5 tests green; app_seeds analyzes clean (0 errors).
