fix(browser): add cross-origin frame enumeration fallback

[YishanCoding]

Summary

• preserve DOM snapshot [F#] order for cross-origin iframes
• lazily fall back from Page.getFrameTree to current-tab-owned Target.getTargets(type=iframe) candidates
• fail closed with frame_enumeration_mismatch for missing, ambiguous, cross-tab, or stale iframe identity
• route eval --frame through the same resolved target

Safety

• fallback runs only when the normal frame tree misses a DOM-observed frame
• candidates must have a CDP parentId chain reaching the current tab target
• duplicate URL/name matches are rejected
• snapshot iframe element identity/order is revalidated before resolution

Tests

• npx vitest run --project extension: 105 passed
• npx vitest run --project unit src/browser/dom-snapshot.test.ts: 34 passed
• npm run typecheck: 0 errors
• npm run typecheck --prefix extension: 0 errors
• npm test: 6743 passed, 1 failed, 1 skipped

The one full-suite failure is unchanged from the pre-change baseline and unrelated: clis/huodongxing/events.test.js > filters Huodongxing loose date results by parsed event date overlap; expected the tomorrow event plus range/same-day, but received only range/same-day.

Validation used mock CDP tests only; no real browser or Judge.me page was opened.