fix(Settings): defer Platform lookup to fix circular dependency startup crash (#56967)

[SnowingFox]

Summary:

Fixes #56967.

Settings.js evaluated Platform.OS at module load time (a top-level if (Platform.OS === 'ios') branch after importing Platform from ../Utilities/Platform). When another module required Settings while Platform.ios.js / Platform.android.js was still mid-initialization (circular require — e.g. NativePlatformConstantsIOS being loaded by Platform.ios.js), the Platform export object was not yet populated, so Platform.OS dereferenced undefined and threw:

TypeError: Cannot read properties of undefined (reading 'OS')
  at Object.OS (Libraries/Settings/Settings.js:21:14)
  at require (.../Utilities/Platform.ios.js:13:1)

This fix defers the platform lookup: Platform is now required lazily inside getSettings() on the first method call, instead of at module load. The public API is unchanged (get / set / watchKeys / clearWatch still delegate to the iOS implementation on iOS and the fallback implementation elsewhere), matching the existing Settings.d.ts.

Changelog:

[GENERAL] [FIXED] - Defer Platform lookup in Settings.js to fix circular dependency startup crash

Test Plan:

Added packages/react-native/Libraries/Settings/__tests__/Settings-test.js (none existed) with 4 tests:

1. the circular-dependency repro from the issue,
2. lazy Platform access (Platform not touched at module load),
3. iOS delegation (get/set/watchKeys/clearWatch call through to NativeSettingsManager),
4. non-iOS fallback behavior.

RED — on the unmodified code, the circular-dependency test fails with exactly the reported bug:

● Settings › should not throw due to circular dependency during Platform initialization
  expect(received).not.toThrow()
  Error name:    "TypeError"
  Error message: "Cannot read properties of undefined (reading 'OS')"
        > 21 | if (Platform.OS === 'ios') {
             |              ^
  at Object.OS (packages/react-native/Libraries/Settings/Settings.js:21:14)
  at require (.../Libraries/Utilities/Platform.ios.js:13:1)
Test Suites: 1 failed, 1 total
Tests:       2 failed, 2 passed, 4 total

GREEN — after the fix:

PASS packages/react-native/Libraries/Settings/__tests__/Settings-test.js
  Settings
    ✓ should not throw due to circular dependency during Platform initialization
    ✓ defers accessing Platform until a method is first invoked
    ✓ delegates get/set/watchKeys/clearWatch to the iOS implementation
    ✓ uses the fallback implementation on non-iOS platforms
Test Suites: 1 passed, 1 total
Tests:       4 passed, 4 total

Regression check — Settings + Utilities suites (Platform lives in Libraries/Utilities):

PASS packages/react-native/Libraries/Settings/__tests__/Settings-test.js
PASS packages/react-native/Libraries/Utilities/__tests__/ReactNativeTestTools-test.js
PASS packages/react-native/Libraries/Utilities/__tests__/codegenNativeComponent-test.js
Test Suites: 3 passed, 3 total
Tests:       15 passed, 15 total

Also verified: eslint on the changed files exits clean (0 errors).

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