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.

Cargo.toml

@@ -14,10 +14,10 @@ rust-version = "1.94.1"
 exclude = [".jj/", ".github/"]
 
 [dependencies]
-donglora-client = "1.0.0"
-iroh = "0.97"
-iroh-gossip = "0.97"
-mainline = { version = "6", features = ["async"] }
+donglora-client = "1"
+iroh = "0.98"
+iroh-gossip = "0.98"
+iroh-gossip-rendezvous = { version = "0.1", path = "../../iroh-gossip-rendezvous" }
 tokio = { version = "1", features = ["full"] }
 clap = { version = "4", features = ["derive"] }
 serde = { version = "1", features = ["derive"] }
@@ -34,10 +34,8 @@ bytes = "1"
 futures-lite = "2"
 rand = "0.9"
 hex = "0.4.3"
-sha2 = "0.10"
 tokio-util = { version = "0.7.18", features = ["rt"] }
 jiff = "0.2.23"
-zeroize = "1"
 
 [dev-dependencies]
 proptest = "1"