A test proves a 400 MB snapshot through a throttled fake store restores within the drill's size-scaled budget #2813

Part of the owner's #2667. Main streams the snapshot into decompression (#2743), records phase times (#2742) and scales the budget with size (#2773). Prove they add up.

Done:

• [ ] test: a fake object store throttled to 150 Mbit/s serves a 400 MB snapshot (generated sparse or compressible, never kept in the repo), and runDrill finishes green within its budget, with each phase's time recorded
• [ ] if it doesn't fit, fix the slowest phase, and the pull names the phase and the time it took
• [ ] the test runs in under 30 s. Scale the size and the speed down together if needed, and say so in the test

Files: test/ for the drill, and src/snapshot/ only if a phase needs fixing.