Add snapshot variants for light/dark, Dynamic Type, RTL and locale

[BarredEwe]

Closes #99

One preview currently produces exactly one rendering. This adds appearance, Dynamic Type, layout direction and locale as snapshot variants.

test_configuration:
  snapshot_variants: [light, dark, accessibilityXXXL, rtl, locale_ru_RU]

#Preview {
    ProfileCard().snapshotVariants([.light, .dark])
}

Implementation

SnapshotVariant covers light, dark, sizeCategory(_:), rightToLeft and locale(_:), and applies each purely through the SwiftUI environment — no UITraitCollection — so the same code path works on iOS, tvOS and macOS.

In the template assertSnapshots now delegates to an assertVariants layer that sits between the device loop and assertSnapshot, so names compose as Name-iPhone 13 Pro-dark. assertSnapshot takes the already-hosted view and preferences, so the default path still renders exactly once and produces byte-identical output when no variants are configured.

.light deliberately has an empty suffix, so a [light, dark] configuration reuses existing reference images and only adds -dark — no one's snapshots are invalidated by upgrading. Other suffixes: -dark, -rtl, -accessibilityXXXL, -locale_ru_RU. An unknown variant name fails loudly, mirroring the existing behavior for unknown devices.

The per-preview modifier is backed by a preference key; when it is absent, the configured list applies.

Tests

Config decoding, makeArguments, and two generator tests (rendered variants array, empty default). On the runtime side, SnapshotVariantTests covers name parsing and suffixes, and three MacOSSnapshotTests cases use a VariantProbe view that encodes the environment as a bit mask in its own width — necessary because the host machine's own dark mode and locale otherwise leak into the assertion.

make test 59/0, make test-cli 22/0, make build clean.

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