tpm-enrol: move NV cert index off OEM-occupied 0x01800001 → 0x01800100

The Dell 820 kept "finding a cert" and bypassing enrolment despite repeated BIOS
TPM clears. tpm2_nvreadpublic revealed 0x01800001 on that hardware is a FIRMWARE
platform index (platformcreate|policydelete|writelocked, 70 bytes — not our
cert; siblings at 0x01800003/4 + 0x01810000). platformcreate survives TPM2_Clear
and policydelete needs the platform's secret policy, so it's permanent and
unusable by us — the blind check false-positived on it, and enrolment could
never write there anyway. Same class of bug as the EK-handle collision.

Move the NV cert index default 0x01800001 -> 0x01800100 (free on the 820, owner
range, overridable via rd.dot1x.nvindex): cmdline (extract-artifacts.sh),
fleet-enrol -nvindex, the dracut hooks, bench scripts, CLAUDE.md invariant #1,
and design/SSP docs. The signing-key handle (0x81000100) is unchanged.

Harden WriteNVCert: refuse a platformcreate index with a clear "pick a free
owner index via rd.dot1x.nvindex" error instead of a confusing undefine
failure. Regression simtest/TestWriteNVCertRefusesPlatformIndex; NV roundtrip +
ReadNVCert tests moved to 0x01800100. All go tests + packer validate pass.

files/enrol/simtest/tpmsim_test.go

@@ -10,6 +10,7 @@ import (
 	"crypto/sha256"
 	"crypto/x509"
 	"crypto/x509/pkix"
+	"strings"
 	"testing"
 
 	"fleet-enrol/tpmkey"
@@ -81,7 +82,7 @@ func TestEnsureKeyAndSignCSR(t *testing.T) {
 
 func TestNVWriteAndPresence(t *testing.T) {
 	tpm := openSim(t)
-	const nv = tpm2.TPMHandle(0x01800001)
+	const nv = tpm2.TPMHandle(0x01800100)
 
 	if tpmkey.NVPresent(tpm, nv) {
 		t.Fatalf("NV should be empty on a fresh TPM")
@@ -229,3 +230,32 @@ func TestEnsureKeyRefusesToEvictProtectedKey(t *testing.T) {
 		t.Fatalf("protected object was evicted (should have been left intact): %v", err)
 	}
 }
+
+// Regression for the Dell 820 finding: our NV index collided with an OEM
+// PLATFORM-created index (platformcreate|policydelete) that no owner clear can
+// remove. WriteNVCert must REFUSE such an index with a clear error (the remedy
+// is a different rd.dot1x.nvindex), not a confusing undefine failure.
+func TestWriteNVCertRefusesPlatformIndex(t *testing.T) {
+	tpm := openSim(t)
+	const nv = tpm2.TPMHandle(0x01800100)
+
+	def := tpm2.NVDefineSpace{
+		AuthHandle: tpm2.TPMRHPlatform,
+		PublicInfo: tpm2.New2B(tpm2.TPMSNVPublic{
+			NVIndex: nv,
+			NameAlg: tpm2.TPMAlgSHA256,
+			Attributes: tpm2.TPMANV{
+				PlatformCreate: true, AuthRead: true, AuthWrite: true,
+				NoDA: true, NT: tpm2.TPMNTOrdinary,
+			},
+			DataSize: 8,
+		}),
+	}
+	if _, err := def.Execute(tpm); err != nil {
+		t.Fatalf("define platform NV: %v", err)
+	}
+	err := tpmkey.WriteNVCert(tpm, nv, make([]byte, 64))
+	if err == nil || !strings.Contains(err.Error(), "platform-created") {
+		t.Fatalf("WriteNVCert should refuse a platform-created index; got: %v", err)
+	}
+}