docs(theseus): record the add-on vault-root derivation as decided

bchwallet keys = HKDF(passwords/0 root, silentmode/addons/bchwallet/mainnet/0)
-> m/44'/145'/0'. Confirmed; must not change once funds exist.

ROADMAP-identity-wallet.md

@@ -22,6 +22,17 @@ m / 1414' / 0'            messenger (Nostr etc.)     ← future
 Same seed, three purposes, no cross-derivation. See
 [DESIGN-password-manager.md](DESIGN-password-manager.md) for the discipline.
 
+**Add-on roots (decided 2026-09-06, do not change).** The vault never stores
+the BIP-39 seed, only per-purpose roots, so add-ons cannot get a fresh
+seed-level purpose after setup. `api.vault.derive("<addonId>/<path>")`
+therefore returns HKDF-SHA256(passwords/0 root, info =
+`silentmode/addons/<addonId>/<path>`). The bundled BCH wallet uses
+`bchwallet/mainnet/0` as its BIP32 master seed and derives `m/44'/145'/0'`
+from it. This is deterministic from the mnemonic (mnemonic → seed →
+passwords/0 → wallet) and works for random-seed vaults too. Changing the
+info string or the parent root would orphan every wallet's funds; recovery
+without Theseus is via the account xprv shown in the wallet's Settings.
+
 ## Strand A — Password manager
 
 ### A.1 — SHIPPED (0.0.3, 9c09955e)