Fix railroad glowing green for non-snapping structures (#4281)

## Problem

When placing a building near a railroad, the railroad glows green to
show the building would snap to it. This should only apply to **City**,
**Port**, and **Factory** — but missile silos, SAMs, and defense posts
(which cannot be placed on railroads) were also triggering the green
highlight.

## Root cause

The core's `overlappingRailroads()` populated snap tiles for *every*
buildable type. In v31 the green highlight didn't leak because the
client renderer (`RailroadLayer.ts`) gated it with a
`SNAPPABLE_STRUCTURES = [Port, City, Factory]` allowlist:

```ts
if (!SNAPPABLE_STRUCTURES.includes(this.uiState.ghostStructure)) return;
```

That guard was lost when the rendering was rewritten into the WebGL
`RailroadPass`, which now unconditionally highlights every tile in
`overlappingRailroads`. The data was always there; only the renderer's
filter was protecting it.

## Fix

Filter by unit type inside `overlappingRailroads()`, mirroring the
existing guard in `computeGhostRailPaths()`. This keeps the
snap-eligible type list defined once in the core (`RailNetworkImpl`) and
fixes the leak regardless of which renderer consumes the data — rather
than re-adding a client-side allowlist a future rewrite could drop
again.

## Tests

Updated `tests/core/game/RailNetwork.test.ts` for the new signature and
added a case asserting `MissileSilo`/`DefensePost`/`SAMLauncher` return
`[]` (and don't even query the rail grid). All 23 tests pass.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

tests/core/game/RailNetwork.test.ts

@@ -178,7 +178,7 @@ describe("RailNetworkImpl", () => {
       };
       (network as any).railGrid = railGridMock;
 
-      const result = network.overlappingRailroads(tile);
+      const result = network.overlappingRailroads(UnitType.City, tile);
 
       expect(railGridMock.query).toHaveBeenCalledWith(tile, 3);
       expect(result).toEqual([42, 45, 50, 60]); // Deduplicated and sorted
@@ -189,10 +189,30 @@ describe("RailNetworkImpl", () => {
       const railGridMock = { query: vi.fn(() => new Set()) };
       (network as any).railGrid = railGridMock;
 
-      const result = network.overlappingRailroads(tile);
+      const result = network.overlappingRailroads(UnitType.City, tile);
 
       expect(result).toEqual([]);
     });
+
+    test.each([
+      UnitType.MissileSilo,
+      UnitType.DefensePost,
+      UnitType.SAMLauncher,
+    ])(
+      "returns empty array for %s which cannot snap to railroads",
+      (unitType) => {
+        const tile = 42 as any;
+        const railGridMock = {
+          query: vi.fn(() => new Set([{ tiles: [50, 42, 60] }])),
+        };
+        (network as any).railGrid = railGridMock;
+
+        const result = network.overlappingRailroads(unitType, tile);
+
+        expect(result).toEqual([]);
+        expect(railGridMock.query).not.toHaveBeenCalled();
+      },
+    );
   });
 
   describe("computeGhostRailPaths", () => {