Housekeeping: hawser compact, memory reclaim, data-dir relocation #64

The disk-space answer, pairing with v0.2's idle-RAM answer.

• hawser compact: stop our own distro (never wsl --shutdown), compact the VHDX (Optimize-VHD where available, diskpart fallback), restart; report bytes reclaimed
• sparse VHDX on new installs where supported
• memory reclaim knobs documented (autoMemoryReclaim guidance; in-distro cache drop on idle-stop is free since the distro terminates)
• hawser config set data-dir <path> relocation: export/move/re-import with checksums, "move it off C:" is a top-3 request

Safety: everything operates on our own distro/VHDX only; compact refuses while containers run (reuses the idle busy-probe).