feat(p2p): ION overlay discovery — service bit + detection + seed flag (#114)

[LiranCohen]

Summary

Third piece of the ION fast-sync overlay (epic #111). Lets ion-nodes find each other over the Bitcoin P2P network so a new node can fast-sync from a peer.

The service bit (rolled, as decreed 🎲)

scripts/pick_service_bit.py picks the bit by dice roll within Bitcoin's experimental range (bits 24–31, reserved for exactly this so it can't collide with NODE_NETWORK/WITNESS/COMPACT_FILTERS/…), five rolls, take the fifth:

rolls (bit positions): [26, 25, 25, 31, 30]
chosen (5th roll): bit 30
ServiceION = 1 << 30 = 0x40000000

What's here

• internal/p2p/overlay.go — ServiceION = 1<<30; SpeaksION(services); Client.IONPeerCount() / IONPeerAddrs() (the discovered overlay neighbourhood among connected peers); an internal speaksION(peer) gate so later phases only send overlay messages to peers that advertised the bit (a vanilla Bitcoin peer is never sent an unknown command).
• internal/p2p/peer.go — advertise ServiceION in our version message (we still serve no Bitcoin services).
• internal/config + cmd/ion-node — --ion-peer (repeatable) → cfg.IONPeers. No public ION seeds exist yet, so they're operator-supplied via the flag for now; the running node we stood up could be the first seed when you're ready.

ServiceION is a placeholder for the wider ecosystem and can change before any real deployment. Dialing/preferring ION peers and the wire protocol land in #115 / #118.

Tests

• SpeaksION over defined + experimental bits — asserts no collision with defined service bits and bit == 1<<30.
• End-to-end handshake: a stub server advertising ServiceION is counted as an ION peer; a vanilla server is not.
• --ion-peer config plumbing; peerConfig advertises ServiceION.

go test -race ./... green (28 packages).

Post-Deploy Monitoring & Validation

• What to watch: an ion-node now advertises service bit 0x40000000 in its version; peers that are also ion-nodes become visible via IONPeerCount(). No behavior change for header/block sync.
• Failure signal / trigger: none expected; advertising an experimental service bit is inert to vanilla Bitcoin peers.
• Window/owner: ongoing once overlay peering (fast-sync 4/7: fast-sync wire protocol (getanchors/anchors, getcas/cas) #115+) uses it.

Part of #111. Closes #114

🤖 Generated with Claude Code