routing: add a real heuristic to public route search

[blackopsrepl]

Problem

The public routing methods call astar with a zero heuristic, which makes the implementation behave like Dijkstra.

Why this matters

This is both a documentation/coherence issue and a performance opportunity.

Scope

• Implement an admissible heuristic for time and/or distance routing.
• Use a configurable max-speed bound for travel-time lower bounds if needed.
• Benchmark route search before/after.
• If heuristic adoption is deferred, update docs to say routes currently use Dijkstra-style search.

Acceptance criteria

• Public route search uses a non-zero admissible heuristic, or docs are corrected.
• Benchmarks demonstrate whether the change helps enough to keep.

[blackopsrepl]

Resolved by #19.

Public route search now uses admissible straight-line lower bounds for both time and distance objectives, so the API no longer falls back to Dijkstra-style zero-heuristic search.