Fix nuke trajectory preview missing SAM interception for would-be-betrayed allies (#4235)

## Summary
Fixes #4226 (Release Blocker, V32 regression).

The WebGL nuke trajectory preview built its SAM threat set by
unconditionally excluding own + allied SAMs
(`BuildPreviewController.updateNukeTrajectoryPreview`). But when the
strike targets allied territory, the alliance breaks at launch —
`NukeExecution.maybeBreakAlliances()` — so the betrayed ally's SAMs
**do** engage the nuke. The preview therefore showed a fully white
trajectory with no intercept X over an allied SAM, even though the bomb
would be shot down (V31 previewed this correctly).

## Fix
- Compute the would-be-betrayed player set with
`listNukeBreakAlliance()` — the exact function the sim uses at launch,
so preview and sim can't drift.
- Keep an allied SAM in the threat set iff its owner is in that set
(extracted as pure `samThreatensNukePreview()`).
- Other (non-betrayed) allies' SAMs remain excluded, matching sim
behavior where only alliances over the blast threshold break.

Both missing artifacts in the issue (red post-intercept segment and X
marker) come from `tSamIntercept` staying at 1.0 because no SAM was
supplied, so this one change restores both.

Cost note: this adds one `circleSearch` per throttled ghost update
(50ms) when the player has allies — same order as the existing
`wouldNukeBreakAlliance` call for the red warning circle.

## Testing
- Unit tests for the new threat-set predicate (4 cases) in
`tests/client/controllers/BuildPreviewController.test.ts`
- `tsc --noEmit`, ESLint, Prettier clean

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

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

tests/client/controllers/BuildPreviewController.test.ts

@@ -1,5 +1,8 @@
 import { describe, expect, test } from "vitest";
-import { shouldPreserveGhostAfterBuild } from "../../../src/client/controllers/BuildPreviewController";
+import {
+  samThreatensNukePreview,
+  shouldPreserveGhostAfterBuild,
+} from "../../../src/client/controllers/BuildPreviewController";
 import { UnitType } from "../../../src/core/game/Game";
 
 describe("BuildPreviewController ghost preservation (locked nuke / Enter confirm)", () => {
@@ -30,3 +33,23 @@ describe("BuildPreviewController ghost preservation (locked nuke / Enter confirm
     });
   });
 });
+
+describe("samThreatensNukePreview (nuke trajectory threat set, #4226)", () => {
+  const allies = new Set([2, 3]);
+
+  test("non-allied SAM threatens the trajectory", () => {
+    expect(samThreatensNukePreview(5, allies, new Set())).toBe(true);
+  });
+
+  test("allied SAM does not threaten when the strike breaks no alliance", () => {
+    expect(samThreatensNukePreview(2, allies, new Set())).toBe(false);
+  });
+
+  test("would-be-betrayed ally's SAM threatens (alliance breaks at launch)", () => {
+    expect(samThreatensNukePreview(2, allies, new Set([2]))).toBe(true);
+  });
+
+  test("other allies' SAMs still excluded when a different ally is betrayed", () => {
+    expect(samThreatensNukePreview(3, allies, new Set([2]))).toBe(false);
+  });
+});