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.

tests/regressions.nim

@@ -112,7 +112,7 @@ suite "ORC service handle lifetime":
     check destroyed == 1
 
 suite "ORC ownership and configuration regressions":
-  test "isEnabled observes a namespace change before any log call":
+  test "isEnabled uses cached namespace thresholds until a refresh":
     var service = initLogService()
     service.setThreshold("worker", lvlError)
     let local = service.threadLocalRef()
@@ -122,8 +122,35 @@ suite "ORC ownership and configuration regressions":
     # Simulate a change from another service handle without introducing a
     # reference-count race into this deterministic configuration test.
     service.setThreshold("worker", lvlInfo)
+    check not logger.isEnabled(lvlInfo)
+
+    local.ensureFreshness()
     check logger.isEnabled(lvlInfo)
 
+    # A stale true result can become false when the logging call refreshes.
+    service.setThreshold("worker", lvlError)
+    check logger.isEnabled(lvlInfo)
+    logger.info("filtered after refreshing configuration")
+    check not logger.isEnabled(lvlInfo)
+
+    # Creating another logger also refreshes their shared local service.
+    service.setThreshold("worker", lvlInfo)
+    check not logger.isEnabled(lvlInfo)
+    discard local.getLogger("another")
+    check logger.isEnabled(lvlInfo)
+
+  test "isEnabled reads the shared root fallback without refreshing namespaces":
+    var service = initLogService(rootLevel = lvlError)
+    let local = service.threadLocalRef()
+    let logger = local.getLogger("worker")
+    check not logger.isEnabled(lvlInfo)
+
+    service.setRootThreshold(lvlInfo)
+    service.setThreshold("worker", lvlError)
+    check logger.isEnabled(lvlInfo)
+    local.ensureFreshness()
+    check not logger.isEnabled(lvlInfo)
+
   test "a failed refresh is retried without another configuration change":
     var control: CloneControl
     var service = initLogService()