How does generational garbage collection work in V8 — the scavenger, mark-sweep-compact, and write barriers — and how does GC pressure show up as frame drops in real apps?

Imagine you have a toy box where you keep your favorite toys on the top shelf (young generation) and old toys on the bottom shelf (old generation). When the top shelf gets too full, you quickly throw away broken toys (scavenger). When the bottom shelf gets messy, you take more time to organize and throw away old junk (mark-sweep-compact). If you spend too much time cleaning, you can't play your game smoothly — that's frame drops!