fix(review): keep the cursor line across a review layout toggle

[barrettruth]

Problem

The review layout toggle carried a file, not a position. Going into the split, the selection handed over was the file plus its hunk index, so the pair opened at the first change of the hunk rather than the line that was selected. Coming back, goto_review_map_selection() looked the file up in the quickfix list and jumped to its entry, which is the diff --git header row, so gs out of a split always landed at the top of that file's diff no matter where the cursor had been.

The mapping needed for the return trip already existed in one direction: a split row resolves to a source line through the pane's alignment rows, which is how opening the source file from a split lands on the right line. Nothing computed the inverse.

Solution

Both directions now travel as a source position, a side and a line number, which is the one thing a review map row and a split row can both name. Entering resolves the map row to that position through the hunk model and moves the pane pair to the row that shows it; leaving reads the position under the split cursor and resolves it back to the map row that carries it, falling back to the file's quickfix row when the row names no source line, such as a file or hunk header. The split position is read when the toggle happens rather than when the split was opened, so moving around inside the split is reflected in the map.

Rows facing a filler, a deletion with nothing opposite it, take their position from the other pane, so deleted lines return to their own map row instead of the nearest surviving line. Duplicate paths across review sections resolve by review key, so a file that appears both staged and unstaged returns to the section it came from.