docs(evm-wallet-experiment): clarify throwaway keyring is ephemeral

Document that throwaway keyrings regenerate their private key on each
`makeKeyring` call (including vat restarts), since baggage only persists
`{ type: 'throwaway' }`. Callers needing stability across restarts must
use `type: 'srp'`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: sirtimid

packages/evm-wallet-experiment/src/lib/keyring.ts

@@ -14,6 +14,11 @@ const harden = globalThis.harden ?? (<T>(value: T): T => value);
 
 /**
  * Options for initializing a keyring.
+ *
+ * Throwaway keyrings are intentionally ephemeral: each call to `makeKeyring`
+ * generates a fresh private key, so the key does not survive a vat restart.
+ * Baggage only persists `{ type: 'throwaway' }`; callers that need key
+ * stability across restarts must use `type: 'srp'`.
  */
 export type KeyringInitOptions =
   | { type: 'srp'; mnemonic: string; addressIndex?: number }