feat(routeviz): route visualizer scaffold — live per-leg route view + /route-mux HTTP seam

[0pcom]

What

A ROUTE-oriented view, distinct from the transport-graph network visualizer (pkg/tpviz). For a chosen (visor, app) it live-renders each active route group's mux legs — the parallel routes to the destination — as a per-leg table: transport kind, next-hop PK, latency, recv/sent rate + share, retransmits, and alive / warm-standby / dead gate state.

This is primarily design + a scaffolded first increment, not a finished visualizer. It proves the live per-leg data pipeline into a browser with no UI rebuild.

Changes

• getRouteMux — new hypervisor HTTP handler exposing RouteGroupMuxInfo at GET /api/visors/{pk}/route-mux?app=<name>. The per-leg mux telemetry was previously reachable only from cli proxy mux info (RPC-only); this is the first browser-reachable seam for live per-leg route data.
• /route-viz — self-contained embedded page (pkg/visor/routeviz/routeviz.html, vanilla JS, no Angular/tpviz build step). Visor + app pickers, live poll, per-leg rows with byte-delta rate/share (the render model from cmd/skywire-cli/commands/proxy/mux_info.go), gate-state pills, per-leg mini-path.
• docs/design/route-visualizer.md — design note: available route data, the two data gaps, the route-selection actuation seam (AddMuxRoute/GrowMuxRoute/RemoveMuxRoute + policy ForwardMux/ReverseMux), and the recommended long-term home.

Scaffolded vs designed-only

Scaffolded (working): the /route-mux HTTP endpoint + the live /route-viz per-leg page + a handler test.

Designed-only (deferred): route-over-graph layout; per-leg full intermediate-PK path (needs pkg/router to record each leg's hop list — out of scope here); route-selection write actions; candidate-route overlay from /route-find+/route-calc; promotion into the Angular routing page.

Recommended home

Standalone /route-viz now (zero build cost, iterable); graduate the selection actuation + path-over-graph layout into the Angular routing page (its RoutingView already has a slot) embedding the tpviz bundle for the graph. Rationale in the design note.

Scope / safety

Read-only and additive. No changes to pkg/router or routing policy. go build . green; go vet + gofmt clean; TestGetRouteViz passes.

Try it

open http://<hv-host>:8000/route-viz     # pick a visor + app, watch legs update live
curl -s http://<hv>/api/visors/<pk>/route-mux?app=skysocks-client | jq
skywire-cli proxy mux info -n skysocks-client --watch 1s   # the CLI render this mirrors