split: keep the companion pane level with the driver's cursor

Scrolling either pane left the other one wherever it happened to be: the
companion only scrolled when the linked row went off-screen (reveal()), so the
link could sit at the bottom edge while the driver's cursor was mid-viewport —
visually incoherent, your eye had to hunt for it.

Add ListingView/DecompView.align(row, screen_row): scroll so the linked row lands
at the SAME viewport offset as the driver's cursor, and use it in _sync_split for
both directions. The two panes now track each other line-for-line, so the eye
reads straight across. Best-effort at the ends (can't scroll above line 0, nor
past the end when the pseudocode is shorter than the viewport).

Pilot split_view: deterministic alignment check — park the listing cursor at a
known viewport offset deep in the function, sync, and assert the decomp's scroll
top is exactly the aligned value (accounting for both clamps). 19/19; full suite
166/2-flake.

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