Mirror default Navigation3 push/pop transitions on iOS in RTL layouts

[Mohamad Jaber (mhammadjaber00)]

The iOS defaultTransitionSpec() and defaultPopTransitionSpec() in NavDisplay.ios.kt slide using the physical SlideDirection.Left/Right, which ignore the composition's LayoutDirection. In an RTL layout (Arabic/Hebrew locales) a push therefore enters from the physical right — exactly as in LTR — while a native UINavigationController push mirrors and enters from the left. To an RTL user the default push animation reads as a back navigation.

This replaces Left/Right with the direction-aware Start/End:

• push: incoming slides toward Start; outgoing slides toward Start with the existing it / 4 parallax and veilOut
• pop: the mirror with End, keeping initialOffset = { it / 4 } and unveilIn

Start/End resolve through LayoutDirection and flip the computed offsets automatically, so the offset lambdas, duration, and easing are untouched. LTR behavior is pixel-identical (Start == Left, End == Right in LTR). defaultPredictivePopTransitionSpec() is already direction-aware via the swipe edge parameter (made RTL-correct for the gesture in #3196) and is not changed. The other platform actuals are unaffected: the Android default is a fade, and desktop/macOS/web use EnterTransition.None / ExitTransition.None.

Adds NavDisplayLayoutDirectionTest in a new iosTest source set for :navigation3:navigation3-ui, asserting the incoming entry's edge for default push and pop under both LTR and RTL (frame-stepped clock, unclipped bounds). The ui-test dependency is wired the same way :navigation:navigation-compose wires project(":compose:ui:ui-test") for its compose tests.

Fixes: https://youtrack.jetbrains.com/issue/CMP-10703/Navigation3-default-push-pop-transitions-on-iOS-are-not-mirrored-in-RTL-layouts

Testing

• Regression test: NavDisplayLayoutDirectionTest (iosTest) — default push/pop enter edges under LTR and RTL. Run on an iOS simulator against the currently published navigation3-ui defaults, both LTR cases pass and both RTL cases fail (push incoming appears a full width off the right edge, pop incoming a quarter width off the left), demonstrating the test catches the bug; with this change all four pass.
• Manually reproduced on a physical iPhone with an Arabic locale: before the change a push enters from the right in RTL; after, it enters from the left, matching UIKit.
• Spec-level verification against Compose Multiplatform 1.12.0: the same ContentTransforms driven through AnimatedContent under LTR and RTL — Left/Right specs animate identically in both directions, Start/End specs keep LTR unchanged and mirror in RTL.

Release Notes

Fixes - iOS

• Default Navigation3 push and pop transitions now follow the layout direction: in RTL layouts a push enters from the left edge and a pop reveals the previous entry from the right edge, matching native UINavigationController behavior