gstack-update-check caches UP_TO_DATE when the remote version could not be read #2786

`bin/gstack-update-check` fetches the remote VERSION (via `git ls-remote` SHA, then the branch-pinned URL fallback) and validates it against `^[0-9]+.[0-9.]+$`. When validation fails it writes `UP_TO_DATE $LOCAL` to the cache (line 256) and exits 0.

Input that passes while broken: both fetch paths fail (no network, no git, blocked raw.githubusercontent.com), or the endpoint returns an HTML error page. `REMOTE` is empty or HTML, the regex rejects it, and a fresh `UP_TO_DATE` line is cached for the 60-minute TTL. Every later run inside the TTL is silent, and an actual upgrade is never surfaced. `browse/test/gstack-update-check.test.ts` (Path G around line 212, Path H around line 223) asserts this as the expected outcome.

Fix: cache `CHECK_FAILED` (or `UNKNOWN`) with a short TTL on the invalid-response branch, and reserve `UP_TO_DATE` for a parsed remote version equal to (or lower than) local.

Same class as #1974 (closed).