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-selftest.sh

@@ -4,7 +4,7 @@
 #
 # Run as root on a booted workstation (the open-port test image is fine — it
 # ships the same tooling). Uses SCRATCH handle/NV and cleans them up, so it never
-# touches the real 0x81000100 / 0x01800001. Pass --use-real-handles to exercise
+# touches the real 0x81000100 / 0x01800100. Pass --use-real-handles to exercise
 # the actual locations instead (still cleaned up unless --keep).
 #
 # It validates, in order:
@@ -24,7 +24,7 @@ for a in "$@"; do case "$a" in
 esac; done
 
 export TPM2TOOLS_TCTI="${TPM2TOOLS_TCTI:-device:/dev/tpmrm0}"
-if [ "$USE_REAL" = 1 ]; then HANDLE=0x81000100; NV=0x01800001
+if [ "$USE_REAL" = 1 ]; then HANDLE=0x81000100; NV=0x01800100
 else HANDLE=0x81008000; NV=0x01500000; fi
 
 WORK=$(mktemp -d); PASS=0; FAIL=0; SKIP=0