fix(navigation): make screen back navigation pop only its own entry

[aoreshkov]

Why

Every screen's back callback called Navigator.goBack(), which pops whatever screen is current. A second call from the same screen could therefore pop the screen beneath it. Two ways that can happen:

• Double tap: the user taps the back arrow twice while the screen is animating out.
• Race: a tap lands at the same moment as the save or delete navigation from refactor(posting): model one-off ViewModel events as UI state #55.

For example, leaving the edit screen could also leave the details screen. At the root of a non-start section (Settings) it was worse. The first call switches to the start section, and the second pops whatever that section was drilled into, which the user never touched.

This was worked out from the code, not reproduced in the app.

What changed

• New method: Navigator.goBack(from: NavKey) acts only while from is the current top entry. Otherwise it does nothing. A second request from the same screen finds a different entry on top and is a no-op.
• Call sites: the posting details and edit entries and the settings entry pass their own route.
• System back: NavDisplay.onBack keeps the plain goBack(), because Navigation 3 calls it once per popped entry.
• CLAUDE.md: the navigation section now records the rule.
• API dump: core:navigation gains one public function.

Considered, not included

dropUnlessResumed on navigating clicks is the pattern in Android's UI events guidance. It works here:

• Navigation 3 limits each entry's lifecycle to STARTED while it animates in or out (release notes, 1.0.0-rc01 and 1.1.0-rc01).
• The JetBrains navigation3-ui 1.1.2 that this project uses does the same in common code.

It complements this change rather than replacing it:

• It also covers double taps on forward navigation (a list row, the edit button), which this change cannot.
• It does not cover navigation triggered by state, such as the save and delete flags, because a LaunchedEffect is not a click.

It touches every navigating click, so it's left for a separate change.

Verification

./gradlew check passes. Five new NavigatorTest cases:

• the top entry pops;
• a repeated call pops only once;
• an entry that is not on top is a no-op;
• an entry in another section is a no-op;
• at a section root, the call returns to the start section only once.

🤖 Generated with Claude Code

[github-actions]

Code Coverage

Overall Project	98.47%	🍏
Files changed	100%	🍏

File	Coverage
Navigator.kt	100%	🍏