Snapshot upper on tmpfs

Put the warm snapshot's overlay upper on tmpfs (RAM) instead of the disk-backed global cache, so warm forks read node_modules from RAM rather than through a disk-backed lower.

Why deferred: The Linux page cache already holds the snapshot tree in RAM after the first read — so repeated forks (the dev loop) hit RAM, not disk. tmpfs would only help the first cold-cache read, and it costs persistent RAM plus the cross-reboot snapshot durability that putting the upper on disk buys.

Revisit when: A profile shows cold-cache fork reads (page cache cold/evicted) are a material fraction of wall-clock in a real loop — only then does trading durability + RAM for the first-read saving pay off.