refactor: extract shared route-result assembly

[blackopsrepl]

Summary

route_snapped and route_with still duplicated the same node-path assembly logic for geometry and totals, and route_edge_snapped had its own copy of the same path traversal work.

This refactor extracts shared helpers for:

• node-path geometry assembly
• node-path distance/time aggregation
• standard route-result construction for time-based node routing

The shared helpers are reused by route_snapped and route_with, and route_edge_snapped now reuses the same geometry and path-metric helpers for its internal graph path portion.

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test

Fixes #11