Faster fallback when the remote CAS cache is unavailable

If the remote compilation (CAS) cache is down or slow, builds used to pay a timeout per request and could stall, since every cache call independently waited for its own failure. This release adds a shared failure tracker: after a few quick failures, subsequent cache lookups and uploads skip the remote backend instantly and fall back to a local compilation. A short-timeout CAS session also surfaces a hung backend in seconds. Caching resumes automatically if the backend recovers mid-build, and this never changes build output: short-circuited loads behave like a cache miss and short-circuited saves skip uploading an already-built artifact.