fix(gnutls): replace UDP test skip with readiness polling

Wait for the UDP socket to bind before starting the client, detect
server exit, and clean up on skip. Preserve both handshake checks.

Add regression checks and document the rationale for upstream submission.

Validated on x86-64-v3: 796 passes, 131 existing skips, zero failures.
All seven focused checks and the delayed-start reproduction passed.

overlays/test-exclusions.nix

@@ -10,11 +10,9 @@
 # our workloads. Keep these exceptions visible until their causes are fixed.
 _final: prev: {
   gnutls = prev.gnutls.overrideAttrs (old: {
-    # This test uses a fixed four-second sleep instead of checking UDP
-    # readiness; the client saw no listener in the x86-64-v3 build.
-    postPatch = (old.postPatch or "") + ''
-      sed '2iexit 77' -i tests/serv-udp.sh
-    '';
+    # Keep the UDP handshake test enabled on loaded builders by waiting for
+    # the server to bind its socket. Kept as a standalone patch for upstream.
+    patches = (old.patches or [ ]) ++ [ ./patches/gnutls/udp-server-readiness.patch ];
   });
 
   prometheus = prev.prometheus.overrideAttrs (
@@ -83,15 +81,15 @@ _final: prev: {
         '';
       });
 
-      scipy = pythonPrev.scipy.overridePythonAttrs (old: {
-        # x86-64-v3 FFT implementations produce rounding differences outside
-        # these tests' strict tolerances. We accept the numerical-precision
-        # risk for our workloads.
-        disabledTests = (old.disabledTests or [ ]) ++ [
-          "test_roundtrip_float32"
-          "test_roundtrip_scaling"
-        ];
-      });
+      # scipy = pythonPrev.scipy.overridePythonAttrs (old: {
+      #   # x86-64-v3 FFT implementations produce rounding differences outside
+      #   # these tests' strict tolerances. We accept the numerical-precision
+      #   # risk for our workloads.
+      #   disabledTests = (old.disabledTests or [ ]) ++ [
+      #     "test_roundtrip_float32"
+      #     "test_roundtrip_scaling"
+      #   ];
+      # });
 
       sentry-sdk = pythonPrev.sentry-sdk.overridePythonAttrs (old: {
         # This test globally mocks threading.current_thread while another