Rival-city routing: right-of-passage and combat routing follow-ups (#371) #1019

Follow-up to #371 ("Units shouldn't route through rival cities") and to review
feedback on PR #1002.

The routing-around-rival-cities behavior is already provided by the
intent-based movement refactor (MapUnit.ResolveIntent), and PR #1002 dropped
its early regression tests rather than merge coverage-only, provisional tests
in that PR. Remaining work, to be affirmed before adding tests:

• Right of passage. The real reported gap (Units shouldn't route through rival cities #371) is players/human units not
  respecting rights-of-passage — i.e. routing through cities of a civ you have
  a RoP with should be allowed, and routing through disallowed city tiles
  should not happen. Fix this in the movement layer.
• Combat routing through enemy cities. Decide the intended behavior for a
  combat unit pathing toward a target beyond an enemy city (at war): must it
  always go around the city, or may it enter the city tile en route to attack
  it? This is part of larger strategic AI (deep targets inside an enemy
  empire) that is not yet affirmed. Adding tests now would bake in assumptions.
• Regression coverage. Once the above are settled, add tests as xUnit
  [Theory] (owner x war-state) to avoid the duplicated test bodies.

Separate from #213 (settler AI re-picking an unreachable destination).