Fix split scroll sync triggered by typing

[PathGao]

Fixes #303

This removes the cursor-position listener from editor-to-preview scroll synchronization. A normal typing edit can move the cursor without representing an editor scroll, so it must not reposition the preview pane. Real editor scroll events continue to synchronize as before.

Regression coverage verifies that the synchronization effect listens to onDidScrollChange and not onDidChangeCursorPosition.

Depends on #302. Merge order: #286 → #287 → #288 → #293 → #296 → #298 → #300 → #301 → #302 → this PR.

Validated with npm ci, npm run check, npm test (135 tests), and cargo test (21 Rust tests).

[PathGao]

@alecdotdev This is the next focused reliability fix in the existing chain. It removes the cursor-change trigger that made split panes jump during normal typing; it retains only actual editor-scroll synchronization.

Please merge after #302, following the chain stated in the PR description.