feat: Frontend test optimization - smart selection, build caching, and flaky test fixes

[bmartel]

Summary

This PR significantly improves frontend test efficiency, stability, and cost-effectiveness through:

• Smart test selection: Skip tests for non-code changes (CSS, docs, configs)
• Build optimization: Production builds with caching instead of dev server
• Parallel execution: Use both CPU cores with cypress-parallel
• Flaky test fixes: Replace arbitrary waits with deterministic state checks
• Test quarantine: Isolate known flaky tests to unblock PRs

Changes

1. Smart Test Selection (select-frontend-tests.sh)

Three-tier detection system that analyzes changed files:

Tier	Action	Examples
Skip	No tests	*.scss, *.css, *.md, *.yml, assets
All Tests	Full suite	core/**, stores/**, package.json
Targeted	Specific tests	Audio changes → audio tests only
Fallback	Smoke tests	Unknown code changes

2. Build & Serve Optimization

Before: Dev server with HMR, compiles on-demand, ~30-60s startup
After: Production build + static serve, ~2s startup, fully cacheable

• Added ci Babel environment for coverage-instrumented production builds
• Added LSF_TEST_MODE flag to load feature flags in production mode
• Build cache invalidates on source, deps, or config changes

3. Parallel Test Execution

Enabled cypress-parallel to use both CPU cores on ubuntu-latest runners:

npx cypress-parallel -t 2 -d "libs/editor/tests/integration/e2e"

4. Flaky Test Fixes

Replaced non-deterministic waits with proper state synchronization:

// Before (flaky)
cy.wait(1000);
VideoView.captureCanvas("canvas");

// After (deterministic)
VideoView.waitForRegionInKonvaByIndex(0);
VideoView.waitForStableState();
VideoView.captureCanvas("canvas");

Fixed tests:

• video/regions.cy.ts
• video/timeline_region_loop.cy.ts
• audio/audio_regions.cy.ts

5. Flaky Test Quarantine

Created tests-yarn-integration-flaky.yml that runs quarantined tests with continue-on-error: true - visible but non-blocking while fixes are validated.

Expected Impact

Metric	Before	After
CSS/SCSS changes	70 min	0 min (skipped)
Docs/config changes	70 min	0 min (skipped)
Full test suite	~35 min	~20 min (parallel)
Retry rate	30-50%	<10%
Build time (cached)	30-60s	~2s

Test Plan

• Verify test selection script correctly categorizes file changes
• Confirm build cache hits/misses work correctly
• Validate flaky tests pass consistently with new waits
• Check coverage still uploads correctly with production builds

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[robot-ci-heartex]

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