docs(wiki): ATECC608 is upcoming — retag ledger signing to the on-host reality

No secure element is on-site: event signing runs on the software HMAC
(EVENT_SIGNING_KEY, an env var on the host disk), so the ledger is
tamper-EVIDENT but forgeable by anyone who owns the host. Several pages
overstated it as present-tense "ATECC608-signed / unforgeable"; correct them.

- NEW concepts/hardware-signer-options.md: four options for a non-extractable
  signing key (USB HSM / YubiKey / reuse the TPM / plain-dongle trap) + the
  recommendation (TPM interim → USB-HSM target; ATECC608 stays for the embedded
  ESP32, wrong part for a PC host).
- entities/atecc608.md: UPCOMING-not-present status banner + PC-vs-embedded.
- disk-os-hardening.md: fix the live-USB row (BIOS boot-order password is
  load-bearing, not Secure Boot — a signed live USB runs); add a physical-tamper
  chain (Dell 7070 CMOS-reset → live-USB → PCR-7 same-signer unseal) + accepted
  risks (that unseal, unsigned-initramfs evil-maid, operator-USB read TODO).
- open-questions #6 reframed; standing-decisions / overview / threat-model /
  index de-overstated; log query entry.

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

wiki/concepts/threat-model.md

@@ -26,7 +26,10 @@ authorised access *through the app*. Encryption does nothing against the classic
 The controls that actually address insider/operator fraud are different in kind:
 
 - **[[append-only-event-chain]]** — events appended, never edited/deleted; a "void" is itself a
-  recorded event, hash-chained, and **[[atecc608]]-signed** (unforgeable).
+  recorded event, hash-chained, and signed. ⚠ Signing is **software today** (key on the host disk) →
+  tamper-*evident* but forgeable by a host owner; a non-extractable **hardware signer**
+  ([[hardware-signer-options]]; [[atecc608]] upcoming) is what makes it truly *unforgeable*. Which is
+  why the load-bearing insider control is reconciliation, next.
 - **[[reconciliation]]** against an authority the operator can't alter — *this is what remote
   sync really is*: a fraud-control mechanism, not just a backup.
 - **[[disk-os-hardening]]** still worthwhile (defeats boot-from-USB) but **not the main event**;