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>

README.md

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+# varde
+
+A small, boring, distro-packageable Linux daemon that owns a
+content-addressed blob store and mirrors content between consenting
+peers, built on [iroh](https://iroh.computer) (iroh-blobs 0.35).
+Applications talk to it over a unix socket: "add this path", "pin this
+hash", "materialize hash X at path Y". Think: what Windows Delivery
+Optimization is for updates — generalized, open, legible.
+
+*varde* (Norwegian): a stone cairn used as a waypoint.
+
+## Design ethos
+
+- **Infrastructure, not product.** No GUI, no self-updater. A daemon, a
+  CLI, a JSON-lines socket protocol, man pages, systemd units.
+- **Legible to the network.** Identifiable mDNS advertisement, DSCP CS1
+  marking, conservative rate limits (10 MiB/s up default), LAN-only with
+  zero WAN upload by default, all transfers stop on metered connections.
+- **Consent-tiered.** Discovery is open; replication is explicit.
+  Content is served only to allowlisted peers, except what you
+  deliberately publish (`--open-lan` pins, exported tickets). All
+  fetched data is BLAKE3-verified regardless — trust gates
+  participation, not integrity.
+
+## Layout
+
+| crate | role |
+|---|---|
+| `varde-proto` | wire types for the socket API (serde, no I/O) |
+| `varde-daemon` | the service: store, endpoint, policy, socket server |
+| `varde-ctl` | CLI client and protocol reference implementation |
+
+`dist/` holds systemd units (system + user, socket activation),
+scdoc man pages, an example config, and an untested Arch PKGBUILD.
+`docs/` has per-milestone decision notes and the redoal integration
+sketch.
+
+## Quick start
+
+```console
+$ varde-daemon --user &
+$ varde-ctl add ~/dataset -r
+added 5b1c…e0 (1234567 bytes)
+$ varde-ctl pin 5b1c…e0
+$ varde-ctl ticket export 5b1c…e0     # hand this to another machine
+$ varde-ctl ticket import <ticket>    # ...which runs this
+$ varde-ctl materialize 5b1c…e0 /srv/dataset   # reflinks when possible
+```
+
+Trusted peers on the same LAN sync overlapping pins automatically:
+
+```console
+$ varde-ctl status          # shows this daemon's node id
+$ varde-ctl peer trust <node-id-of-the-other-machine>   # on both ends
+```
+
+## Building and testing
+
+```console
+$ cargo build --release
+$ cargo test --workspace          # spawns real daemons; no mocked iroh
+$ cargo clippy --workspace --all-targets -- -D warnings
+```
+
+The `metered` feature (default on) needs D-Bus at runtime only; build
+with `--no-default-features` for systems without it.