fix(navigation): guard forward navigation against repeat taps

[aoreshkov]

Follow-up to #56, which made back navigation pop only its own entry. This PR covers the forward direction with two complementary guards.

Problems

1. Duplicate entries in the two-pane layout. Tapping the selected posting row again (or the add/edit FAB twice) pushed a second entry with the same key. It shares the first entry's content key, so it also shares its ViewModel and saved state. Nothing visibly changes, and the user then has to press back twice to leave.
2. Taps during a single-pane transition. On Android and iOS the outgoing screen stays composed and clickable while the scene animates. Tapping a second row during the slide pushed another screen on top.

Changes

• Navigator.goTo is single-top: a no-op while the destination is already the current section's top entry. It is a pure back-stack check, so it works in every layout and at any timing.
• dropUnlessResumed wraps the three forward-navigation clicks (posting row, add FAB, edit FAB), following the Android UI-events guidance. Navigation 3 caps entries in a transitioning scene at STARTED, so the second tap is dropped.

dropUnlessResumed alone would not fix problem 1. With two panes, ListDetailSceneStrategy keeps one scene key for the whole list-detail stack, so NavDisplay never transitions and every entry stays RESUMED. In turn, goTo single-top alone cannot catch problem 2, where the destinations are different. Back arrows keep goBack(from) from #56; wrapping them as well would be redundant.

On desktop, NavDisplay's default transition is None, so the single-pane window does not exist there. Only the single-top check matters on desktop.

Tests

• NavigatorTest: repeated goTo(same) adds once; the same key below the top still appends; goTo(sectionRoot) is a no-op.
• Screen tests host each screen at STARTED and assert the click is dropped (list row, add FAB, edit FAB). Mutation-checked: removing one guard fails exactly one test.
  • The test owner uses LifecycleRegistry.createUnsafe. On desktop, the regular registry's main-thread check runs runBlocking(Dispatchers.Main), which deadlocks in test classes that swap Main for a StandardTestDispatcher.
• Run locally, all passing: :core:navigation:check, :feature:posting:impl:check, and posting jvmTest and testAndroidHostTest.

No public API change (the goTo signature is unchanged), so no apiDump.

🤖 Generated with Claude Code

[github-actions]

Code Coverage

Overall Project	98.48%	🍏
Files changed	100%	🍏

File	Coverage
Navigator.kt	100%	🍏
PostingDetailsScreen.kt	98.49%	🍏
PostingListScreen.kt	98.42%	🍏