Add buildMu to serialize replaceFileUsers's writer body (including
buildEntry's potentially slow file-backed resolveSecret reads for
"file:"-prefixed API keys). Scope mu.Lock() to the final byDigest
swap only, so a rebuild in progress no longer stalls every identify
call for its whole duration. Behavior-preserving: writers stay fully
serialized, same as before.
Adds a regression test that holds buildMu externally and asserts
identify still succeeds — proving identify's read path does not
depend on the writer-serialization lock.
lukaszraczylo