feat(router): N-hop BFS in calculateLocalRoutes — deterministic local routing

[0pcom]

Summary

Operator's observation: the route-finder service is the dominant source of variance in our mux-bw measurements (26× throughput swing across intermediates for an identical request shape). Local route calc is deterministic by construction — same input, same output. But until now `calculateLocalRoutes` only supported 1-hop (direct) and 2-hop routes; anything deeper fell through to the route-finder service (and its randomness).

This PR extends `calculateLocalRoutes` to find N-hop paths via BFS over the transport-discovery graph, bounded by `Config.MaxHops`. Combined with `--local-route` on the caller, the operator gets a deterministic selector:

```
cli visor ping mux-bw --local-route --min-hops 3
→ same 3-hop path every call, picked deterministically by
remote-PK string sort at each BFS expansion step.
```

Combined with `DialOptions.ExcludeIntermediatePKs` (#2746), the operator can run controlled N=2/4/8 disjoint-mux experiments where each route is reproducibly chosen — the per-intermediate variance becomes selectable rather than random.

Implementation

• Drop the previous "if MinHops > 2: error out" guard (was fix(router): calculateLocalRoutes must honor MinHops > 2 #2753's plug)
• Drop the previous 2-hop-only intermediate search loop
• BFS state: `{ pk, path []Hop }`, level = path length
• Seed queue with `localTps` (1-hop paths to direct neighbors), excluding any in `ExcludeIntermediatePKs`
• At each level, check if any queued node is dst with path ≥ MinHops; return shortest-acceptable
• Expand each node by walking `transportsByEdge[node.pk]`, skipping already-visited (no cycles)
• Deterministic ordering: sort seed and each level's children by remote-PK string before queueing

`reverseHops` + `hopPath` helpers added for forward-route flipping and log-line rendering.

Backwards compatibility

• Callers without `DialOptions.MinHops` set get `minHops=2` (same as previous behavior)
• 1-hop direct fast path above the BFS is unchanged
• Self-ping 2-hop loop above the BFS is unchanged
• Only N≥3 path discovery is new

Test plan

• `go build ./...` clean
• `golangci-lint run` clean
• `go test ./pkg/router/...` pass (37s)
• Live: `mux-bw --local-route --min-hops 5 ` finds a 5-hop path without route-finder service
• Live: same call repeated returns same path (determinism)
• Live: `--local-route --routes 4 --min-hops 2` with DisjointMux yields 4 routes with disjoint intermediates