Much better river handling for nations and tribes! 🏞️ (#3786)

## Description:

In this example, the two nations DONT see each other as neighbors, but
as ISLANDERS. Because they dont have a direct border connection, there
is water in between.

<img width="526" height="329" alt="image"
src="https://github.com/user-attachments/assets/cf2c15b5-7793-4445-afd2-920d6cd50a2a"
/>

This is a big problem, because most of the logic in AiAttackBehavior
gets ignored. Only the "islander" strategy runs (late, because its a
not-important strat).

### Summary

- `PlayerImpl.neighbors()` now includes cross-water neighbors: a new
`shoreReachableNeighbors()` helper samples every 10th shore border tile
and looks up to 5 tiles in each cardinal direction across water, finding
land owners on the other side (covers rivers up to 4 tiles wide).

- `AiAttackBehavior.maybeAttack()` extends the `hasNonNukedTerraNullius`
check to also trigger on TN detected via `player.neighbors()`, so
nations notice and pursue TN that is only reachable across a river.

- `sendAttack()` uses a new `hasLandBorderWithTerraNullius()` land-only
adjacency check to decide between a land attack and a boat attack for
TN, rather than `sharesBorderWith()` which includes water tiles.

- Added `sendBoatAttackToNearbyTerraNullius()`: when no TN land is
directly adjacent, the AI scans its shore border tiles for unowned land
across water and dispatches a transport ship.

### Also works for Tribes!

Tribes can boat rivers now, really cool.


https://github.com/user-attachments/assets/382e85aa-c437-4e0c-afc2-0c381432da3d

## Please complete the following:

- [X] I have added screenshots for all UI updates
- [X] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [X] I have added relevant tests to the test directory
- [X] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

FloPinguin

tests/NationStructureBehavior.test.ts

@@ -47,7 +47,7 @@ function makePlayer(
 ): any {
   return {
     units: vi.fn(() => ownUnits),
-    neighbors: vi.fn(() => neighborList),
+    nearby: vi.fn(() => neighborList),
     canTrade: vi.fn((n: any) => opts.canTrade?.(n) ?? true),
     isOnSameTeam: vi.fn((n: any) => opts.isOnSameTeam?.(n) ?? false),
     isAlliedWith: vi.fn((n: any) => opts.isAlliedWith?.(n) ?? false),