Exclude editor bottom padding from split scroll sync

[PathGao]

Fixes #315

Monaco keeps blank space below the final line because scrollBeyondLastLine is enabled. The previous synchronization range used scroll height, so that non-content space made the editor’s final document line map to a preview position above its end.

Uses Monaco content height for both directions of split-scroll synchronization while retaining the editor’s scroll-beyond-last-line preference and front-matter segmentation.

Adds a regression assertion that synchronization does not use the padded scroll range.

This is stacked after #314 and its predecessors. Merge the predecessor chain first.

Validated with npm run check (0 errors, 0 warnings), npm test (136 passing), npm run build, and cargo test (21 passing).

[PathGao]

@alecdotdev This is the follow-up for the remaining end-of-document drift reported in #205. It keeps Monaco’s normal bottom-padding preference but removes that padding from the sync calculation. It is stacked after #314 and the predecessors; please review in order.