test(desktop): make PTT HAL liveness deterministic

[Git-on-my-level]

Fixes #10724.

What changed and why

• wait until the injected CoreAudio probe has actually entered its wedged semaphore before checking responsiveness
• replace the fixed > 10 timer/read throughput requirements in a 300 ms window with three explicit main-actor heartbeat completions and three explicit currentSnapshot completions
• keep snapshot reads off-main so a blocking regression times out without preventing the test from releasing and joining the parked HAL probe
• exempt the real Swift package test path, desktop/macos/Desktop/Tests/, from the user-facing changelog gate and cover that path classification with regression cases

testMainRunLoopKeepsPumpingWhileTheHALIsWedged was testing a binary liveness contract with host-throughput thresholds. Under macOS runner contention, the main actor and snapshot path still made progress, but the isolated suite completed too few iterations. The production contract is that the main actor and turn-start snapshot reads both keep completing while the HAL probe remains blocked off-main, so the new test observes those two completions independently instead of measuring scheduler throughput.

The changelog gate already exempts internal test paths because its post-merge push run cannot see a PR-only no-changelog-needed label. It covered the legacy lowercase desktop/macos/tests/ directory but not the Swift package tests under Desktop/Tests/, so this test-only patch exposed the same open failure class on the actual Swift test path.

Failure evidence

• current main: run 30276084659, reads == 10
• unrelated fix(ci): make pre-push portable on Windows #10574 head: run 30280746072, ticker.count == 5, reads == 4
• unrelated fix(ci): make pre-push portable on Windows #10574 rerun: run 30283186121, ticker.count == 7, reads == 7

All sibling PTTInputDeviceProbeTests cases passed in those failures.

Product invariants affected

none

How it was verified

• python3 .github/scripts/test_desktop_changelog.py passed: 4 tests
• python3 .github/scripts/check-desktop-changelog.py --base origin/main --head HEAD passed: no desktop changes require a changelog entry
• python3 desktop/macos/scripts/check_desktop_test_quality.py passed
• git diff --check origin/main...HEAD passed
• reviewed the semaphore ordering: started is fulfilled immediately before release.wait(), main-actor heartbeat and snapshot-read expectations settle before release.signal(), and finished is awaited afterward

This contribution was prepared on Windows, so the local commit hook could not run the Xcode-only swift-format command (xcrun is unavailable). Desktop Swift CI is the authoritative compile, formatter, and XCTest verification for the Swift test change.

Tests

• ChangelogRequirementTests now covers both a top-level and nested Swift package test path.
• The rewritten PTT test is the regression proof for the three linked flaky runs; it fails in bounded time if either the main actor stalls or snapshot reads block while the injected HAL probe is parked.

Failure class (fixes)

Failure-Class: FC-push-gate-internal-path-scope

Scoped cleanups

The changelog path exemption is a separate commit because the initial test-only head demonstrated that relying on no-changelog-needed would pass only the PR lane and could still redden the post-merge push lane.