(fix) preserve extension sidebars during transitions

[Ammaar-Alam]

Summary

• limit question-transition concealment to the active AAMC review surface
• keep unrelated extension roots laid out and the MKit rail host pointer-transparent
• preserve the fail-closed full-page cover when the AAMC surface cannot be isolated
• clean up transition markers across normal review, teardown, and startup failures
• cover repeated native transitions and shadow-sidebar wheel scrolling in Chrome

Context

The reporter later confirmed that Issue #6 was caused by a different extension, so this PR does not claim to fix that report. During the investigation, live tracing found a separate MKit defect: every AAMC question transition temporarily hid all sibling body roots and made MKit's full-viewport host hit-testable. This change fixes that verified interoperability problem.

Verification

• pnpm check
• 143 unit tests passed
• 59 Chrome tests passed
• lint, typecheck, build, and release audit passed

(PR written by Codex)