Let the wheel scroll inner scroll containers in comment mode

[mssh21]

Summary

• In comment mode the fixed selection cover receives every wheel event, so the browser only scrolled the cover's ancestors (the document). Pages that scroll inside an element could not be scrolled while commenting: an app shell under html, body { overflow: hidden } froze completely, and a scrolling table under the pointer stayed still while the document moved instead. This was reported on a React project, but the cause is the layout (CSS), not React.
• The cover now hands the wheel to the nearest element under the pointer that can still scroll in that direction, through open shadow roots and accessible same-origin frames. The top document is still left to the browser so its native smooth scrolling is kept, and Ctrl+wheel (pinch or page zoom) is untouched.
• docs/architecture.md describes the behaviour.

Checks

• Reproduced in the React demo (demo/react.html) before the fix: with #root as the scroll container, the wheel moved nothing in comment mode (it scrolled 500 px outside comment mode); over the accounts table, the table stayed at 0 and the document scrolled.
• After the fix in the same demo: #root scrolls, the table scrolls to its end, and clicking a cell after scrolling selects that cell.
• New spec the wheel in comment mode scrolls a page whose content scrolls inside an app shell instead of the document (@cross-engine) failed in Chromium and WebKit before the fix and passes after it.
• pnpm typecheck, pnpm test (512 passed), pnpm build, pnpm test:e2e (297 passed).

Notes

• Touch scrolling on the cover is unchanged: the cover keeps touch-action: none so touch drags select an area.

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