fix(P11): SVID chain validation (REQ-126, F9)

---ci---
project: orca
phase: 11
milestone: v0.12
status: execute
---/ci---

VerifySVIDWithChain: validates the full cert chain against the CA pool
+ checks the SPIFFE URI SAN. Rejects certs from unknown CAs even with
correct URI (F9). VerifySVID retained for backward compat (mTLS
callers that already verified the chain). 2 new tests. Build + vet green.

.ciagent/CHECKPOINT.json

@@ -1,16 +1,16 @@
 {
-  "phase": 5,
+  "phase": 10,
   "stage": "complete",
   "milestone": "v0.12",
   "milestone_slug": "security-hardening",
   "phase_role": "execution",
   "attempts": 0,
-  "updated_at": "2026-08-07T11:03:00Z",
+  "updated_at": "2026-08-07T11:19:00Z",
   "milestone_complete": false,
   "previous_milestone": "v0.11",
-  "wave": "B (P06 ACL rewrite, P07 password removal, P08 master key seal) next",
-  "phases_shiped": ["P0","P1","P2","P3","P4","P5"],
-  "tags_shipped": ["v0.11.0","v0.11.1","v0.11.2","v0.11.3","v0.11.4","v0.11.5"],
+  "wave": "C (P11 SVID chain, P12 backup symlink) next",
+  "phases_shipped": ["P0","P1","P2","P3","P4","P5","P6","P7","P8","P9","P10"],
+  "tags_shipped": ["v0.11.0","v0.11.1","v0.11.2","v0.11.3","v0.11.4","v0.11.5","v0.11.6","v0.11.7","v0.11.8","v0.11.9","v0.11.10"],
   "binding_conditions": ["C-29","C-30","C-31","C-32","C-33","C-34","C-35","C-36","C-37","C-38"],
   "phase_count": 29,
   "load_bearing_rule": "R-021"

internal/identity/spiffe.go

@@ -99,6 +99,47 @@ func VerifySVID(certPEM []byte, spiffeID string) error {
 	return fmt.Errorf("identity: cert missing %q: %w", spiffeID, ErrSpiffeURIMissing)
 }
 
+// VerifySVIDWithChain validates the SVID cert chain against the CA
+// pool AND checks the SPIFFE URI SAN (REQ-126, F9). The CA pool is the
+// cluster root CA (or the step-ca root). Rejects certs signed by
+// unknown CAs even with a correct URI. This is the hardened
+// verification path; VerifySVID (above) only checks the URI and is
+// retained for backward compatibility (callers that have already
+// verified the chain via mTLS).
+func VerifySVIDWithChain(certPEM []byte, spiffeID string, caPool *x509.CertPool) error {
+	if caPool == nil {
+		return fmt.Errorf("identity: VerifySVIDWithChain requires a non-nil CA pool (REQ-126)")
+	}
+	block, _ := pem.Decode(certPEM)
+	if block == nil {
+		return fmt.Errorf("identity: parse cert: PEM decode failed: %w", ErrStepCLI)
+	}
+	cert, err := x509.ParseCertificate(block.Bytes)
+	if err != nil {
+		return fmt.Errorf("identity: parse cert: %w", err)
+	}
+	// Verify the cert chain against the CA pool.
+	if _, err := cert.Verify(x509.VerifyOptions{
+		Roots: caPool,
+		// SVIDs are client certs (workload identity); they don't have
+		// EKU for serverAuth, so we use the default (any EKU).
+		KeyUsages: []x509.ExtKeyUsage{x509.ExtKeyUsageAny},
+	}); err != nil {
+		return fmt.Errorf("identity: SVID chain validation failed: %w (REQ-126: unknown CA or expired)", err)
+	}
+	// Check the SPIFFE URI SAN.
+	want, err := url.Parse(spiffeID)
+	if err != nil {
+		return fmt.Errorf("identity: parse spiffe id: %w", err)
+	}
+	for _, u := range cert.URIs {
+		if u.String() == want.String() {
+			return nil
+		}
+	}
+	return fmt.Errorf("identity: cert missing %q: %w", spiffeID, ErrSpiffeURIMissing)
+}
+
 func SpiffeIDFromCert(cert *x509.Certificate) string {
 	for _, u := range cert.URIs {
 		if u.Scheme == "spiffe" {

internal/identity/spiffe_test.go

@@ -240,3 +240,27 @@ func TestSanitize(t *testing.T) {
 		t.Errorf("sanitize = %q, want %q", got, want)
 	}
 }
+
+// --- REQ-126 / F9 SVID chain validation tests ---
+
+// TestVerifySVIDWithChain_RejectsUnknownCA verifies a cert from a
+// wrong CA is rejected.
+func TestVerifySVIDWithChain_RejectsUnknownCA(t *testing.T) {
+	// Generate a cert signed by a different CA (not the pool's CA).
+	certPEM := mintTestSVIDCert(t, "spiffe://orca.local/ns/test/sa/web/alloc-1")
+	// Empty CA pool (no trusted roots).
+	emptyPool := x509.NewCertPool()
+	err := VerifySVIDWithChain(certPEM, "spiffe://orca.local/ns/test/sa/web/alloc-1", emptyPool)
+	if err == nil {
+		t.Error("VerifySVIDWithChain should reject cert from unknown CA (REQ-126)")
+	}
+}
+
+// TestVerifySVIDWithChain_NilPoolRejected verifies nil CA pool errors.
+func TestVerifySVIDWithChain_NilPoolRejected(t *testing.T) {
+	certPEM := mintTestSVIDCert(t, "spiffe://orca.local/ns/test/sa/web/alloc-1")
+	err := VerifySVIDWithChain(certPEM, "spiffe://orca.local/ns/test/sa/web/alloc-1", nil)
+	if err == nil {
+		t.Error("nil CA pool should error (REQ-126)")
+	}
+}