security: verify registration proof-of-possession (PPA-003 server side)

[TeoSlayer]

Completes the server half of PPA-003. The client-signing half is pilot-protocol/common#37; the daemon already signs register:<listen_addr>:<public_key> with the same ed25519 identity it uses for heartbeat/deregister.

HandleRegister now verifies the optional signature field over register:<listen_addr>:<public_key> against the submitted public_key, using the same crypto.Verify primitive as the existing signed operations. The verify runs lock-free (before HandleReRegister takes s.mu), per the file's lock-ordering rule.

Backward-compatible by default (verify-if-present):

• unsigned registrations from old agents are still accepted;
• a present-but-invalid signature is rejected — catches tampering / forged re-registration endpoint-repoints even in the default mode;
• new -strict-registration-auth / RENDEZVOUS_STRICT_REGISTRATION_AUTH requires a valid signature (opt-in; enable only once the fleet signs). Mirrors the existing default-off -strict-directory-auth (WS2) wiring exactly.

End-to-end compatibility verified: daemon signer is base64(ed25519.Sign(identity, challenge)) (web4 daemon.go:2260); this verify is ed25519.Verify(pub, "register:<addr>:<pubkey>", sig) — identical challenge + primitive.

Tests: TestHandleRegisterSignatureVerification — 7 cases (unsigned-allowed-by-default, unsigned-rejected-when-strict, valid-accepted both modes, wrong-key-rejected both modes, malformed-rejected). Full go test -race ./... green (20 pkgs), no regressions.

Deploy note: ship with the flag off, let the signing client roll out fleet-wide, then flip -strict-registration-auth for the airtight guarantee.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!