test: cover cancel dialog lifecycle failures

[jackhai9]

Summary

• expand the deterministic cancel-current-symbol fixture with missing, malformed, delayed, and React-replaced native dialogs
• cover Escape, backdrop, BFCache, real pagehide, symbol change, delayed clearing, and incomplete clearing
• assert order isolation and exact restoration or deliberate recovery-state preservation for every lifecycle outcome

Validation

• npm run test:binance-orderbook-trade (218/218)
• npm run test:ui (17/17)
• git diff --check
• syntax checks for all browser-test JavaScript

Scope

Test fixture and regression scenarios only. No userscript source, generated artifact, or version change.