ac-os: portable cache size check in pull_ota
`stat -c%s` is Linux-only; on macOS it silently failed and the fallback
`|| echo 0` made the SLIM/INITRAMFS cache check always miss, forcing a
~340MB re-download on every `ac-os pull`. Swap to `wc -c <` which works
on both platforms.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>