Milestone 6: DiscoveryProvider seam, signed announcements, redoal sketch

The discovery module defines the seam: DiscoveryProvider (subscribe/
announce over 32-byte TopicKeys) and a signed Announcement carrying
root hash, ed25519 author, metadata and provider addresses. Signatures
cover a deterministic postcard encoding including the topic (no cross-
channel replay) and the provider identities (addresses stay refreshable
hints). LanDiscovery conforms to the trait — mdns sightings become
locally-authored announcements, one per pinned root — and the daemon's
auto-sync now runs entirely through it: verify, index unconditionally,
fetch only for trusted authors from allowlisted providers on already-
pinned incomplete roots. The milestone-4 real-mdns sync test passes
unchanged through the new path. Verification unit tests cover round
trip, tampered root, wrong topic, forged author, mismatched signing
key, and serde survival. docs/redoal-integration.md sketches the
gesture-topic gossip provider against this contract.

Also: fix a flaky hang in the socket-activation test (dup2(3,3) leaves
CLOEXEC set when the listener already sits on fd 3; parent's listener
copy masked daemon death), and give the test client a read-timeout hang
guard. Add a top-level README.

Dependencies: ed25519-dalek (Signature type; same implementation iroh
keys use), postcard (deterministic signed encoding, iroh's canonical
compact codec).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/milestone-6.md

@@ -0,0 +1,37 @@
+# Milestone 6 — Discovery provider seam
+
+## What landed
+
+- `varde_daemon::discovery` with the spec's `DiscoveryProvider` trait,
+  `TopicKey`, and the signed `Announcement` type. Signatures are ed25519
+  (the same key type as iroh NodeIds) over a deterministic postcard
+  encoding that includes the topic, so announcements can't be replayed
+  onto other channels. Verification tests cover round trip, tampered
+  root, wrong topic, forged author, mismatched signing key, and
+  serde-then-verify.
+- `LanDiscovery` implements the trait: mDNS sightings become
+  locally-authored, signed announcements ("peer N may hold pinned root
+  R"), one per pinned root. `announce()` is a no-op on the LAN — mDNS
+  already advertises presence, and varde never broadcasts content lists.
+- The daemon's auto-sync was refactored *onto* the seam: sightings feed
+  presence tracking and the LanDiscovery channel; a single
+  `on_announcement` path verifies, indexes, and applies the consent
+  tiers (trusted author → pinned + incomplete → trusted providers →
+  fetch). The milestone-4 mdns integration test passes unchanged through
+  the new path, which is the proof the seam carries real traffic.
+- Announcements surface on the event stream (`announcement` events) and
+  are indexed in memory per root.
+- `docs/redoal-integration.md` sketches the gesture-topic gossip
+  provider against this contract, including the one extension it will
+  need (topic-scoped trust) and why nothing else changes.
+
+## Deviations from the spec sketch
+
+- The trait deals in `SignedAnnouncement` rather than bare
+  `Announcement`: the spec says announcements are signed, so the wire
+  type carries its signature and the subscriber can't forget to check.
+- `Announcement.author` is an iroh `PublicKey` (ed25519, as specified);
+  provider addresses are excluded from the signed payload (they're
+  refreshable hints) while provider identities are covered.
+- Signature bytes travel as a length-checked `Vec<u8>` because serde
+  cannot derive for `[u8; 64]`.