fix(canvas): drop a card where it snapped, and save a note on the way out

Alignment snapping settled from a fresh computation at release rather than from
the frame the user was looking at. Same inputs, but the live Alt state and zoom
are read again — so tapping or releasing Alt after the final mouse move handed
back a different correction than the one still on screen, and the card jumped
on mouseup, which is the mismatch snapping exists to remove. The drop replays
the last painted correction instead.

That drop was also classified from the raw pointer position while the card had
visibly come to rest up to a snap tolerance away — exactly enough to file it on
the wrong side of a region edge. It takes the grabbed node from the snapped set
now, so where a card lands and where it is filed are the same place.

A note's unsaved text is mirrored from its change handler rather than synced in
an effect. The unmount save exists for the ways out that never fire blur — a
view switch, ReactFlow culling the node, a remote delete — and a passive effect
carrying the last keystroke may still be queued when the node goes away, which
drops precisely the character the save was there for. `commit` clears the
mirror synchronously, so a blur immediately followed by an unmount can't send
the same patch twice.