Fix iOS LaunchImage Contents.json mixing Any-scale + specific scales

[barrymortenson]

Fixes #141.

What

InstallsSplashScreen::installIosSplashScreen() builds LaunchImage.imageset/Contents.json from whichever of six hard-coded splash variants exist in public/. The first two (splash.png, splash-dark.png) have no scale key, so Xcode treats them as "Any" scale. When @2x and/or @3x variants also exist, the generated Contents.json has a mix Xcode's asset compiler rejects:

error: Image set has a child with bitmap content and the "Any" scale.
The image set also has children with specific scales and content.
Please remove the content with specific scales or change the content
of the "Any" scale to vector content.
** ARCHIVE FAILED **

Simulator builds skip GenerateAssetSymbols so the bug stays invisible until a release archive runs the asset compiler. That made it survive in our project until Bifrost's first App Store build hit it.

How

If any scaled variant exists, drop the unscaled ones from the candidate set before the existence loop:

$hasScaledVariants = false;
foreach (['splash@2x.png', 'splash-dark@2x.png', 'splash@3x.png', 'splash-dark@3x.png'] as $scaledFilename) {
    if (File::exists(public_path($scaledFilename))) {
        $hasScaledVariants = true;
        break;
    }
}
if ($hasScaledVariants) {
    unset($splashVariants['splash.png'], $splashVariants['splash-dark.png']);
}

Behavior matrix

public/ contains	Before	After
only splash.png (± -dark)	one "Any"-scale entry (valid)	unchanged — one "Any"-scale entry (valid)
only @2x and @3x variants	scaled-only entries (valid)	unchanged — scaled-only entries (valid)
all six variants	mixed Any + scaled (invalid → archive fails)	scaled-only entries (valid)

The 1x splash files are effectively ignored when scaled variants are present. iOS handles a missing 1x by downsampling from 2x; no currently-supported iOS device uses 1x scale anyway (the original iPhone is the only 1x device and is unsupported by modern iOS).

Alternative considered

I considered just adding 'scale' => '1x' to the two unscaled entries in $splashVariants. Simpler diff, but it changes the meaning for projects that ship a single splash.png and rely on it being the "Any" fallback across densities — scale: 1x would constrain it to 1x devices only. The conditional-skip approach in this PR avoids that behavior change.

Test plan

• Manually verified with all six variants: generated Contents.json now contains only @2x and @3x entries; archive succeeds.
• Manually verified with only splash.png (no scaled variants): generated Contents.json contains the single unscaled entry; archive succeeds.
• No existing iOS splash unit tests in this repo (tests/Unit/Traits/InstallsAndroidSplashScreenTest.php covers Android only). Happy to add an InstallsIosSplashScreenTest in a follow-up if you'd like — the Android test gives a clear pattern to follow.