fix(review): work around Safari scroll-reset bug with shadow DOM diffs

[backnotprop]

Summary

• Works around a Safari/WebKit compositor bug where scrollTop resets to 0 when momentum-scrolling ends inside a scroll container with a <diffs-container> shadow DOM child
• Adds a WebKit-only scroll event listener that detects bogus resets (scrollTop jumping from >200 to 0 in a single event) and synchronously restores the last known good position
• Zero cost on Chrome/Firefox/Edge — the listener is never attached outside WebKit

Root cause

Safari's compositor loses track of scroll position when a web-component shadow DOM (@pierre/diffs <diffs-container>) is the child of a scroll container. The reset bypasses all JavaScript APIs — intercepting scrollTo() and the scrollTop setter caught zero calls. Confirmed via systematic elimination:

1. Not OverlayScrollbars — bug persists with a plain <div style="overflow:auto">
2. Not the CSS layout — a static tall div scrolls fine in the same flex container
3. Not any JS code — intercepting scrollTo and scrollTop setter caught nothing
4. It IS @pierre/diffs — removing the <diffs-container> shadow DOM fixes scrolling

Test plan

• Open a code review with a large diff in Safari — scroll down with trackpad momentum, verify position is maintained
• Scroll up to the top in Safari — verify it reaches the top normally without being blocked
• Switch files in the file tree while scrolled down — verify new file starts at the top
• Verify scrolling works normally in Chrome/Firefox (guardian is inactive)

Closes #540