Make known Nim allocator race scenarios opt-in

Skip factory-state and allocator by default with explicit skip messages. Preserve both scenarios and enable them with NSL_TEST_RUNTIME_ALLOCATOR=1. Document the deferred runtime issue and remaining validation limits.

Validation: nimble test passes 32 unit tests, 29 regression tests, and 11 sanitizer scenarios, with two skips. Opt-in execution still reports both known allocator failures.

namespaced_logging.nimble

@@ -43,7 +43,7 @@ proc runThreadSafetyTests() =
     raise newException(OSError, "Writer lifecycle test compilation produced no executable")
   exec "bash tests/run_tsan.sh tests/bin/thread_safety tests/bin/writer_lifecycle"
 
-task test, "Run all tests, including ThreadSanitizer, and report all failing suites.":
+task test, "Run all enabled tests, including ThreadSanitizer, and report all failing suites.":
   var failedSuites: seq[string] = @[]
   template runSuite(name: string, action: untyped) =
     echo "Running test suite: ", name