router: add DialOptions.OnProgress hook for route-setup progress observability

[0pcom]

Problem

pkg/router DialOptions exposes no per-hop / noise-handshake progress observer. A caller that dials a route can only see the terminal result (success / failure) — not the intermediate "found 2-hop path via X → establishing route group → noise handshake → up" progression.

This came up building the streaming proxy interstitial (#4054): the chunked-streamed interstitial can only show a coarse per-attempt progress line + reload-on-success, because the granular route-setup narrative is not observable. The stream infra fabricates nothing — it surfaces only the signals that exist. The mux telemetry harness likewise only synthesizes coarse RouteEstablished/failed events around its own DialRoutes.

Proposal

Add an optional OnProgress func(RouteSetupEvent) callback to DialOptions (and the route-group/DialRoutes path), emitting structured events as setup proceeds:

• route(s) found — candidate paths, hop count, intermediary PKs
• route-group establishment start / per-leg
• noise handshake start / complete / deadline-exceeded (per leg)
• rule installation
• final up / failed (with reason)

Opt-in, nil-safe (no callback = today's behavior), no data-plane cost.

Consumers this unblocks

1. Streaming interstitial (feat(interstitial): stream live route-setup progress via chunked encoding #4054) — the real "2-hop via X → handshake → up" play-by-play instead of per-attempt guesses.
2. Route visualizer — live route-setup animation + failure surfacing.
3. Per-proxy status pages (feat(proxy): per-proxy status hosts + HTTPS interstitial permit-gate #4050/feat(proxy-status): serve status over browse-origin real cert + AA contrast #4053) — the live "is-my-connection-up" liveness signal folded into the mux view.
4. Route-drop diagnosis (Implement visor restart from hypervisor #80) — the same hook makes the intermittent noise-handshake-deadline directly observable at the dial site rather than only in scattered logs.

Notes

Separate from the interstitial/status work already merged; this is the missing router-side seam those features want. Keep it a pure observability addition — no behavior change when the callback is unset.