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.

docs/fleet-management-design.md

@@ -20,7 +20,7 @@ These are consequences of the CLAUDE.md invariants, not preferences:
    stateless and buffers only in RAM (best-effort).
 2. **Identity from the TPM, never baked.** No per-host state in any artifact
    (invariant #1). The agent derives identity at runtime from the **TPM-held
-   cert** (key `0x81000100`, cert NV `0x01800001`) — the same identity used for
+   cert** (key `0x81000100`, cert NV `0x01800100`) — the same identity used for
    802.1X EAP-TLS and the NVMe hostnqn. No enrollment token in the image.
 3. **Pull-based control, no inbound surface.** The boot LAN is locked down and
    segmented; we do not open an always-on inbound control port on hosts. Hosts