fix(breath): clamp negative stage durations - #1436

What this PR does

Aligns Android breath-stage duration handling with Swift by normalizing negative stage durations to zero at construction time.

The Kotlin model remains immutable and retains its data-class value semantics. Mirrored Swift and Kotlin regression tests verify the stored duration, zero cycle duration, and empty scheduling result for a negative stage.

This addresses only the negative-duration parity finding in bhelm#32. The other findings in that issue remain out of scope.

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Documentation
• CI / tooling

How it was tested

• cd Packages/StrandAnalytics && swift test --filter BreathProtocolPlayerTests — 7 tests passed
• cd android && ./gradlew :app:testFullDebugUnitTest --tests "com.noop.analytics.BreathProtocolPlayerTest" --no-daemon --max-workers=1 --no-parallel — 7 tests passed

Only the focused suites were run. No manual, device, or strap testing was performed; the behavior is covered by deterministic unit fixtures.

Checklist

• Swift package tests pass for any package I touched (swift test in Packages/<name>) — focused BreathProtocolPlayerTests passed; the full package suite was not run
• Android unit tests pass if I touched android/ (./gradlew testFullDebugUnitTest) — focused BreathProtocolPlayerTest passed; the full Android suite was not run
• No new build warnings introduced
• UI changes use only StrandDesign tokens — not applicable; no UI changed
• No hardcoded hex frame bytes; protocol facts live in the schema / decoders — not applicable; no protocol code changed
• Follows the conventions in docs/CONTRIBUTING.md
• I did not commit generated output (Strand.xcodeproj/) or any secrets/keystores

Related issues

Related: bhelm#32