Pull the inline leading+trailing resize throttle out of useMainApp into
createResizeCoalescer (src/lib/resizeCoalescer.ts) and cover it directly
with fake-timer tests: leading-edge immediacy, burst collapse to one
trailing reflow, fresh leading edge after the window, cancel() dropping a
pending reflow, and sustained-drag staying ~one reflow per interval.
Also seed lastReflow at -Infinity instead of 0 so the leading edge fires on
the first event independent of the wall clock (the inline version only
worked because Date.now() is large at runtime).
671b1b058e