Two residual gaps in the #67369 salvage of #67214, found during review:
1. No timeout on the download client. Since #67369, mutable branch pins
hit the network on EVERY run, and the stale-cache fallback only fires
when download() returns Err. A black-holed connection (captive portal,
hung proxy, dropped packets) never errors, so the whole bootstrap hung
at resolve() instead of falling back to the cached script. Verified
live: a request to a non-routable address now errors at the 10s connect
timeout instead of hanging indefinitely.
2. The UTF-8 BOM was only written inside download(). Immutable commit-pin
caches take CachePlan::Reuse and are served untouched forever, and the
stale-fallback path also re-serves the old file - so a BOM-less .ps1
cached by a pre-fix installer kept reproducing the #67193 ANSI-codepage
parse failure on every retry (production builds pin BUILD_PIN_COMMIT,
so this is exactly the retry population). upgrade_cached_script() now
BOM-upgrades legacy .ps1 caches in place (atomic tmp+rename,
best-effort, idempotent) on both reuse paths; .sh untouched.
73b3a8afe3