Give macOS scrolling momentum

A swipe that was still moving when the fingers lifted stopped dead. The dial
next to it has thrown properly since pardes_rotate_end, and this is that same
curve on the scroll accumulator: velocity sampled between events off the
monotonic clock, weighted toward the newest sample because a flick is decided
by how the hand was moving when it left, and a fling that ramps up from zero
at the floor rather than switching on at it.

A coast stops at the end of a document rather than spinning its remaining
velocity against the edge, which is why spendScroll now reports whether the
pane moved and why paneAt is public. Only a step that delivered a press can
report an edge — the many steps between two rows cross nothing.

Nothing suppresses AppKit's own momentum, and nothing needs to: its momentum
events are ordinary pardes_scroll calls, every one of which cancels the coast
before spending its travel, so on a real trackpad the system takes the gesture
over about a frame after the lift and the tail it ends on is below the floor.
This is the path for devices AppKit does not fling for — and the only one a
script can reach, since NSEvent phases have no public constructor.

momentum.snap asserts both halves, which is why it needs the long scrollback:
a flick with no document left proves nothing. A slow swipe is byte-identical
after the release; a hard one coasts twenty-four rows further on its own, and
a finger back on the pad stops it there.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

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