ccmux v0.1.18

fix: 'ccmux update --check' is now read-only and a stale/older staged bundle can't silently downgrade a machine

• Fix ccmux update --check mutating the machine. With a leftover staged bundle present (a forgotten
  bun run stage), --check applied it instead of just reporting — and applied it even when it was
  OLDER than the running version, silently downgrading. Two root fixes: (1) the update decision is now
  a pure decideUpdate that --check can only ever ask for a print from — read-only by
  construction; (2) a staged bundle wins only when NEWER-or-equal than the running version (an
  unreadable one counts as not-newer) — a stale/older staged build is refused as a downgrade unless
  --force, with a message pointing at the forgotten file. The legit "test a newer build locally"
  path is unchanged.