refactor(core): co-locate the quic wire tests with their modules

quic/tests.rs (1813 lines, 43 tests) was the W7 split's leftover: the
source moved into handshake/caps/control/clock/pairing/pake/datagram/
endpoint/clipstream/io but every test stayed in one monolithic file.
Each test now lives in a #[cfg(test)] mod tests at the foot of the
module it exercises, verbatim. The two CompositorPref/GamepadPref
wire/name tests moved to config.rs (where those enums live), so they
now also run under --no-default-features. The clip_loopback and
ctrl_framing integration mods share connect_pair via a cfg(test)-only
quic/test_util.rs.

Test-only motion: 196 lib tests pass unchanged on macOS, clippy
--features quic --all-targets clean, include/punktfunk_core.h
byte-identical.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-core/src/quic/endpoint.rs

@@ -293,3 +293,16 @@ impl rustls::server::danger::ClientCertVerifier for AcceptAnyClientCert {
             .supported_schemes()
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::quic::endpoint;
+
+    #[test]
+    fn fingerprint_is_sha256_of_der() {
+        // Stable across calls, distinct for distinct certs.
+        let a = endpoint::cert_fingerprint(b"cert-a");
+        assert_eq!(a, endpoint::cert_fingerprint(b"cert-a"));
+        assert_ne!(a, endpoint::cert_fingerprint(b"cert-b"));
+    }
+}