fix(tls): honor Go key exchange defaults across transports

Remove the eight explicit curve overrides so Go 1.27 honors tlsmlkem=0
across Server listeners, node links and outbound transports. Remove the
unused shared curve option and add wire-level regression coverage.

Document CA trust and TLS upgrade behavior, retain the investigation
artifacts, and exclude their synthetic routes from the rebrand guard.
The product compatibility baseline remains unchanged.

Validation: focused race tests, HTTP tests, lint, compatibility guard
positive/negative controls, and a fresh Linux build with three isolated
OIDC integration scenarios all pass.

Adversarial review: Claude Code Fable 5.1, max effort.
Final verdict: APPROVE FOR COMMIT.

Signed-off-by: Feng Ruohang <rh@vonng.com>

docs/investigations/issue-154/cert-roots.go

@@ -0,0 +1,41 @@
+//go:build ignore
+
+// Run only with the public, synthetic CA made by fixture.go.
+package main
+
+import (
+	"crypto/x509"
+	"encoding/json"
+	"encoding/pem"
+	"os"
+	"runtime"
+
+	"github.com/pgsty/silo-pkg/v3/certs"
+)
+
+func main() {
+	if len(os.Args) != 3 {
+		panic("usage: cert-roots synthetic-ca.pem explicit-ca-path-or-empty")
+	}
+	data, err := os.ReadFile(os.Args[1])
+	if err != nil {
+		panic(err)
+	}
+	block, _ := pem.Decode(data)
+	if block == nil || block.Type != "CERTIFICATE" {
+		panic("expected public certificate")
+	}
+	certificate, err := x509.ParseCertificate(block.Bytes)
+	if err != nil {
+		panic(err)
+	}
+	roots, err := certs.GetRootCAs(os.Args[2])
+	if err != nil {
+		panic(err)
+	}
+	_, err = certificate.Verify(x509.VerifyOptions{Roots: roots})
+	_ = json.NewEncoder(os.Stdout).Encode(map[string]any{
+		"go": runtime.Version(), "os": runtime.GOOS,
+		"explicit_ca": os.Args[2] != "", "trusted": err == nil,
+	})
+}