fix: include transitDetails in Routes API field mask

[cablate]

Summary

Closes #78

The COMPUTE_ROUTES_FIELD_MASK in src/services/RoutesService.ts was missing routes.legs.steps.transitDetails, so transit steps returned only a free-text navigationInstruction.instructions (e.g. "Bus towards Sean Moore Rd") without structured metadata — bus/route number, stop names, headsign, agency, stop count. Google Routes API only returns this object when explicitly requested via the X-Goog-FieldMask header.

This also resolves the symptom reported in #74 (comment) — "transit directions tell you to take a bus, but doesn't describe which one" — which shares the same root cause.

Changes

File	Change
src/services/RoutesService.ts	Add routes.legs.steps.transitDetails to COMPUTE_ROUTES_FIELD_MASK
tests/smoke.test.ts	New testTransitDetailsField: assert transit step contains transitDetails with transitLine/stopDetails

Impact

• Additive only. The field is absent in non-transit steps, so driving/walking/bicycling responses are unchanged.
• No schema or call-site changes. PlacesSearcher.getDirections already passes routes through verbatim; maps_directions ACTION does JSON.stringify(result.data), so the new object flows through untouched.
• No effect on computeRouteMatrix (distance matrix) — its field mask has no steps level.

Test plan

• npm run build succeeds
• npm run lint — 0 errors (175 pre-existing warnings unchanged)
• npm test — 198 passed, 0 failed
• Verified live API response: Dublin Airport → Trinity College Dublin returns a bus step with transitDetails.{stopDetails, transitLine, headsign, stopCount, localizedValues}

Root cause verification

Independently called the Routes API with the new field mask:

Dublin Airport -> Trinity College Dublin: step[0] "Bus towards Dublin Airport Stops"
  transitDetails keys: stopDetails, localizedValues, headsign, transitLine, stopCount

The same field mask addition also restores structured transit metadata for Harold's Cross → Rathmines (the exact route in #78) and Paris → Lyon.