feat(airc): add signing_pubkey_hex to AircPeerManifest (L1-6 verify substrate)

[joelteply]

Summary

Adds the missing signing_pubkey_hex field to AircPeerManifest so the L1-6 contract event chain can do peer_id → pubkey lookups at verify time. The substrate's trust answer becomes "the manifest IS the directory" — no separate keyring, no out-of-band cert exchange.

• Closes kanban card: 290f64b7-5837-42ff-9844-570088fbb01a
• Builds on: L1-4 feat(airc): add peer manifest capability index #1446 (AircPeerManifest + capability index, just merged)
• Unblocks: L1-6 Phase B (peer_id→pubkey lookup at envelope verify time)

Why

L1-6 #1448 shipped Phase A — ed25519 signing + 8-event contract chain + envelope verify. verify() returns the pubkey that signed; the caller must cross-check it against an external trust source to confirm signer identity matches proposer_id / bidder_id / etc.

That cross-check needs a peer_id → pubkey directory. L1-4 #1446 landed AircPeerManifest (peer_id, capabilities, room_ids, timestamps) but no pubkey. This PR completes that surface.

What this lands

• AircPeerManifest.signing_pubkey_hex: String — required field. 32-byte ed25519 public key, hex-encoded (64 chars, no 0x prefix). Matches SignedContractEvent::signer_pubkey_hex byte-for-byte — same encoding, no transcoding when L1-6 Phase B parses one for verify.
• AircPeerManifest::validate() — validates the field structurally (length + hex chars). Curve-membership / point-on-line is delegated to ed25519_dalek when a consumer parses the bytes.
• AircPeerManifestError enum (EmptyPeerId / PubkeyWrongLength / PubkeyNonHexChar) — specific variants so the inbound L1-2 subscriber can log + reject with actionable diagnostics rather than a generic "bad manifest". Per the never-swallow-evidence rule.
• ts-rs auto-export updates shared/generated/airc/AircPeerManifest.ts with the new field as required signingPubkeyHex: string.

Why required (not optional)

AircPeerManifest is a new type — L1-4 just landed yesterday. No existing manifest traffic to migrate. Making the pubkey required from day one means it's structurally impossible to advertise a peer without a verifiable identity. Optional now → "trust me, my next manifest will have one" later → forever optional. Required is the correct default for a substrate trust directory.

Key rotation

Doc-commented on the type: a peer that mutates its own pubkey publishes a fresh manifest; receivers that already have one for that peer_id reject the mismatch loud. Key rotation has to go through a proper trust-rotation event class, not silent overwrite. Tracking the rotation event class is L4 work; the no-silent-overwrite stance is set today.

Tests (6 new + all 38 realtime tests pass)

• validates_well_formed_pubkey
• accepts_uppercase_hex — substrate must NOT reject otherwise valid uppercase just for case
• rejects_wrong_length_pubkey
• rejects_non_hex_pubkey
• rejects_empty_peer_id
• round_trips_through_json_with_pubkey — verifies camelCase wire form (signingPubkeyHex) + serde round-trip

Bonus fold-in

Generated TS barrel shared/generated/airc/index.ts picks up 3 backfill entries (AircCapabilityIndexEntry, AircPeerCapability, AircPeerManifest) that L1-4's generator pass missed. Pure barrel re-export drift; harmless to include here.

Follow-up

L1-6 Phase B PR will add ContractVerifyingKey::from_hex (or similar) so the verify path reads signing_pubkey_hex straight from the manifest into the verify primitive without re-encoding.

Test plan

• cargo test --features metal,accelerate airc::realtime — 38 tests pass
• CI build + clippy ratchet

🤖 Generated with Claude Code