dsh-win32 v0.5.3

Proactive-audit release, before anyone hit these.

1. Spill contract restored. The shell executors request spill files on collect-mode spawns, and the v0.5 decoding collector silently produced none, so large command outputs on win32 lost their full-stream spill. The collector now streams raw bytes to a spill file exactly like stock (created on first overflow, discarded past the cap, raw semantics identical, and GBK spills were raw in stock too).
2. done waits for pipe drain. A read immediately after process exit could miss final chunks. The handle's outcome now settles only after the claimed pipes drain.
3. busybox download fallback. If frippery.org is unreachable (some networks), setup now says how to pass a manually downloaded binary (--busybox <path>), and the README carries a China-network note for both raw.githubusercontent.com and frippery.org.

27 unit tests, full matrix green.