Add support for impassable terrain

[Billytifft]

Closes #362

Summary

Adds the Impassable terrain flag from the BIQ TERR section to the terrain model and makes unit movement respect it. This fixes terrains like the WWII Pacific scenario's desert, which has a movement cost of 0 but must block entry entirely.

ImpassableByWheeled is deliberately left for a follow-up, as scoped in the issue.

Changes

• TerrainType: new impassable property, imported from civ3Terrain.Impassable
• Tile: IsImpassable() helper, reading the overlay terrain (consistent with MovementCost())
• MapUnit.ResolveIntent: returns Disabled on impassable tiles, which gates every entry path:
  • manual movement (MapUnit.Move)
  • go-to pathing (A* CanEnterPeacefully/CanEnterForcefully)
  • go-to execution (MoveAlongPath)
  • go-to UI (goto cursor / hover cost)
• Tests: MakeImpassableDesertTile() helper plus two tests — a land unit can't enter an impassable tile (even forcefully) and can't path through one

Verification

• dotnet build C7/C7.sln — 0 errors
• dotnet test C7/C7.sln with CIV3_HOME set — 69/69 pass, 0 skipped (real Civ3 scenario/save tests included)
• dotnet format C7/C7.sln whitespace --verify-no-changes — clean

Standard BIQs have no impassable terrain, so this is a no-op for normal games; it only affects mods that flag terrain impassable.

[ajhalme]

#362 explicitly mentions 0 movementCost. That could be a good test case to check.

[ajhalme]

LGTM, test is fine as is

[stavrosfa]

I think barbarians spawn on these impassable tiles, if I remember correctly from the ww2 map.
So perhaps only human units are unable to move there, or, it's just barbarians that can move on them.
Obviously this got merged, so perhaps an issue could be opened and linked to this PR as reference, so we dont miss it in the future?