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.

serving/dot1x-bench/enrol/tpm-provision.sh

@@ -7,7 +7,7 @@
 #      (non-reserved owner handle; NOT the EK slot 0x81010001)
 #   2. build a CSR using that TPM key (private key never leaves the TPM)
 #   3. have step-ca sign it (token-gated 'enrol' provisioner)
-#   4. write the issued cert (DER) to TPM NV index 0x01800001
+#   4. write the issued cert (DER) to TPM NV index 0x01800100
 # At boot the dracut hook reads the cert from NV and wpa_supplicant signs EAP-TLS
 # with the persistent-handle key. hostnqn is later derived from the cert serial.
 #
@@ -27,7 +27,7 @@ set -euo pipefail
 
 HOSTID=${1:?usage: tpm-provision.sh <host-id>   e.g. ws-vm-a}
 HANDLE=${HANDLE:-0x81000100}
-NVINDEX=${NVINDEX:-0x01800001}
+NVINDEX=${NVINDEX:-0x01800100}
 export TPM2TOOLS_TCTI="${TPM2TOOLS_TCTI:-device:/dev/tpmrm0}"
 CA_DNS=${CA_DNS:-ca.fleet.lab}; CA_PORT=${CA_PORT:-8443}
 CA_URL="https://${CA_DNS}:${CA_PORT}"