QA: Add visual regression baselines or geometry assertions for visual polish

[ChuxiJ]

Problem

Visual regression tests capture screenshots but do not compare baselines or assert layout geometry. CI can pass even when visual polish regresses.

Root Cause

Screenshot capture was added as a manual artifact workflow, not as an automated pass/fail visual regression gate.

Solution

• Add baseline screenshot comparison for stable surfaces, or geometry assertions for dynamic surfaces.
• Cover timeline playhead, clips, mixer, generation panel, plugin panels, and status bar.
• Document when screenshots are artifact-only vs blocking assertions.

Verification

• Run npx playwright test tests/e2e/visual-regression.spec.ts.
• Intentionally perturb a measured layout locally and verify the test fails.
• Confirm CI stores useful artifacts for failures.

Files to Touch

• tests/e2e/visual-regression.spec.ts
• tests/e2e/__screenshots__/*
• docs/qa/*