feat!: factor DHT rendezvous into iroh-gossip-rendezvous 0.1

The partition-healing / DHT-rendezvous algorithm that used to live in
`src/gossip.rs` is now its own crate, published as
`iroh-gossip-rendezvous` 0.1.0. Bridge depends on it for swarm
discovery; the 760-line gossip module shrinks to ~75 lines of glue.

`dht-partition-healing.md` is reduced to a pointer at the new
crate's `PROTOCOL.md` — the normative spec moved with the code.

### Dependency changes

- **New:** `iroh-gossip-rendezvous = "0.1"` (path + version for local
  dev; publish strips the path).
- **Bumped:** `iroh` and `iroh-gossip` 0.97 → 0.98 (major).
- **Removed:** direct deps on `mainline`, `sha2`, `zeroize` — all
  now transitive via `iroh-gossip-rendezvous`.
- `donglora-client` spec loosened from `"1.0.0"` to `"1"` so patch
  and minor releases of the client propagate automatically.

BREAKING CHANGE: any direct users of `bridge::gossip` internals
relied on the in-bridge partition-healing types that have moved.
Import from `iroh_gossip_rendezvous::…` instead.

dht-partition-healing.md

@@ -0,0 +1,11 @@
+# Moved
+
+The DHT partition-healing algorithm specification is now maintained as part
+of the `iroh-gossip-rendezvous` crate:
+
+**<https://github.com/swaits/iroh-gossip-rendezvous/blob/main/PROTOCOL.md>**
+
+The bridge pins a specific `iroh-gossip-rendezvous` version; the spec that
+matches whatever version you're running is in
+`~/.cargo/registry/.../iroh-gossip-rendezvous-*/PROTOCOL.md` or, for path
+dependencies, at `../iroh-gossip-rendezvous/PROTOCOL.md`.