Two cache-key correctness follow-ups to #82229 (review feedback):
1. Abbreviated commit pins are immutable too. The installer's
is_valid_commit() accepts 7-40 hex chars, but the Python refresh
exempted only exactly-40-hex names — an abbreviated pin like
install-4ce1994.ps1 could be overwritten with a branch script. The
predicate now mirrors the Rust rule (7-40 hex = immutable, never
rewritten), applied to the sanitized target ref.
2. Refresh only the update-target ref's cache key. The helper rewrote
EVERY mutable-ref entry with the active checkout's script: with
install-main.ps1 and install-bb_gui.ps1 coexisting, updating main
replaced both with main's script — cross-branch cache poisoning in
the other direction. It now computes the single cache key for the
branch being updated, using the installer's own ref sanitization
(sanitize_ref: non [A-Za-z0-9._-] -> '_', so bb/gui ->
install-bb_gui.ps1), and touches nothing else. Entries the
bootstrapper never wrote are not created.
The branch is threaded from the existing `branch =
_resolve_update_branch(args)` in both _cmd_update_impl call sites and
_update_via_zip (main-only by its own guard).
Regression tests lock down both invariants: abbreviated-SHA pin
untouched (including when passed as the branch), coexisting mutable
refs (main refresh leaves install-bb_gui.ps1 byte-identical),
sanitize_ref parity, and uncached-ref no-op.
E2E on the incident machine's real bootstrap-cache: planted a stale
install-main.ps1 + sibling install-bb_gui.ps1 + abbreviated pin
install-4ce1994.ps1; refresh("main") healed main byte-exact and left
both others untouched; refresh("4ce1994") was a no-op. The pre-existing
40-hex pin entry in the real cache was also untouched.
