test: push coverage to 72% — 902 tests across 41 files

New test files (6):
- ActionExecutor.test.ts (38 tests) — click/type/navigate/scroll execution
- SessionManager.test.ts (52 tests) — session lifecycle, tabs, security guard
- BrowserManager.test.ts (30 tests) — browser launch, context, stealth config
- VisionGate.test.ts (24 tests) — visual regression, baselines, SSIM, OCR, heatmaps
- index.test.ts (5 tests) — barrel exports verification
- presets.test.ts (12 tests) — built-in profile structure and values

Coverage: 58% -> 72% statements (+14%)
Tests: 708 -> 902 (+27%)
Files: 35 -> 41

Author: AVANT-ICONIC