Define cached threshold behavior for isEnabled

Document and test explicit refresh boundaries for isEnabled guards, and
only read the shared root threshold when no namespace threshold applies.
Include the portable Nimble configuration loader.

config.nims

@@ -0,0 +1,4 @@
+# begin Nimble config (version 2)
+when withDir(thisDir(), system.fileExists("nimble.paths")):
+  include "nimble.paths"
+# end Nimble config