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/dracut/95dot1x-macsec/dot1x-macsec-up.sh

@@ -31,7 +31,7 @@ if [ -z "$NIC" ]; then
         NIC=$_name; break
     done
 fi
-NVINDEX=$(getarg rd.dot1x.nvindex=); [ -n "$NVINDEX" ] || NVINDEX=0x01800001
+NVINDEX=$(getarg rd.dot1x.nvindex=); [ -n "$NVINDEX" ] || NVINDEX=0x01800100
 TIMEOUT=$(getarg rd.dot1x.timeout=); [ -n "$TIMEOUT" ] || TIMEOUT=60
 
 info "dot1x-macsec: using interface $NIC, NV index $NVINDEX"