Complete extension SDK maturity

extensions/sdk-fixture/INTENT.md

@@ -0,0 +1,13 @@
+# SDK Fixture Extension
+
+`sdk-fixture` is a dependency-light guide-board extension used to exercise the
+extension SDK contracts. It is intentionally small and is not a real assessment
+program.
+
+The fixture demonstrates:
+
+- extension-owned target and assessment profile schemas,
+- a Python runner that writes native output,
+- a Python normalizer that converts native output into guide-board evidence,
+- a mapping set for normalized requirement refs,
+- copyable profiles for SDK acceptance tests.