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/README.md

@@ -84,7 +84,7 @@ qm set <vmid-A> --tpmstate0 <storage>:1,version=v2.0
 3. **Enrol each workstation's TPM** (one-shot, on a provisioning network that can
    reach step-ca) — see `enrol/tpm-provision.sh`: generates the key **in** the TPM
    at `0x81000100`, builds a CSR with it, has step-ca sign it, writes the cert to
-   NV `0x01800001`. Each host gets a unique cert → unique hostnqn.
+   NV `0x01800100`. Each host gets a unique cert → unique hostnqn.
 
 4. **Set up the authenticator** — `authenticator/setup-link.sh` (bridge + macsec
    plumbing per link) then run `hostapd authenticator/hostapd-wired-<link>.conf`.