Find a paired machine again after either side changes network

[Tryanks]

Closes #409.

Behaviour

A pairing is bound to the machine identity (host_id), not to an address. When the saved origin stops answering, or this device's interface addresses change, a native client races every origin it knows for the same machine and resumes the same session without re-pairing or manual address entry.

• PairedHost gains a bounded, ordered candidates list (max 16): origins that completed hello before, the addresses the machine reports in hello_ok, and mDNS hints. Older hosts.json records (one origin, or legacy addrs+port) still load; the one-origin form re-serializes unchanged.
• hello_ok now carries addrs and port; hello_rejected carries host_id. A hello_ok or token rejection from a different machine at a stale address counts as unreachable, never as "Pair again". Only the paired machine (or an older host at the saved origin that does not name itself) can end the pairing.
• Reconnect racing (250 ms stagger, 5 s budget, max 32 origins) covers saved candidates, mDNS hints, hotspot gateways (iOS/Android/macOS/Windows hotspot subnets), the .1 of private networks up to /24, and every guest of a private /28 or smaller. Only for plain http origins; https pairings are never downgraded. The browser client keeps its fixed page origin.
• The native connection loop polls the local interface set (2.5 s while reconnecting, 10 s while connected). A change cuts the backoff short with a fresh candidate list, or probes a live socket within 3 s.
• Backoff resets its delay on a network change while the attempt counter keeps counting, so the banner and the store's per-attempt browse dedup keep working.
• Host side: mdns-sd 0.21.3 already re-announces addr_auto services on interface changes (ip-check timer every 5 s), so the beacon needs no change beyond the hello fields. The listener already binds all interfaces.

The transport is now the single owner of "find the host again"; the UI store only forwards mDNS hints through Wake::Candidates (iOS can drive Bonjour only from the UI thread). refresh_origin is replaced by discover_origins.

Tests

• Literal older hosts.json fixtures load and migrate; promotion and bounded candidates.
• hello_verdict against literal hello_ok/hello_rejected shapes: mismatch is non-terminal, missing host_id primary-vs-candidate, addrs filtering.
• Interface probe derivation and interface-set change detection (pure functions).
• Integration (two real servers): a stranger at the saved address rejecting the token stays Reconnecting, the answering candidate is promoted, token and stamp untouched; literal hello_ok/hello_rejected JSON asserted.
• Backoff: network change shortens the next delay without restarting attempt numbers.

Checks run

cargo fmt --all --check, cargo clippy --workspace --all-targets --locked -- -D warnings, cargo build --workspace --locked, cargo test --workspace --locked (all green), cargo machete, iOS sim / wasm32 / Android arm64 cargo check with -D warnings.

Live check: headless host paired on port 47500, then restarted on 47501 with the same data dir; the client hit connection-refused on the saved origin, picked up the mDNS hint, promoted the new origin in hosts.json and reconnected with the original token (one device record on the host, no re-pair). Not exercised live: real phone hotspots and interface aliasing (covered by pure-function tests).

Known gaps

• When this device hosts a /24 hotspot, the machine's guest address cannot be guessed; mDNS or the machine's reported addresses must supply it.
• CGNAT (100.64/10) and other non-private networks get no gateway probe by design.
• Promotion (transport thread) and stamp_connected (UI) both load→modify→save hosts.json; they run sequentially in practice but there is no file lock.
• Racing sends the bearer token to guessed LAN origins over plain HTTP, the same exposure as the previous mDNS-hint origin swap, widened to gateway/subnet probes.