test: known-answer coverage for HD derivation and the vault (closes #159)

wallet.js and vault.js — the two modules that hold user funds — had no
derivation or encryption tests. Add them, pinned to published vectors
rather than to whatever the implementation returns today.

wallet.js: hdWalletFromMnemonic, hdWalletFromXprv, deriveAddressFromXpub
and getSignerForAddress are pinned to the standard development recovery
phrase's first three accounts at m/44'/60'/0'/0/n and to the BIP-39
all-zero-entropy phrase's first address; addressFromPrivateKey is pinned
to the published key/address pairs, so the HD path and the bare-key path
must meet at the same address from two directions. isValidMnemonic and
isValidXprv cover bad checksum, wrong word count, wrong key type and
empty/garbage input. The absolute-vs-relative path asymmetry between
hdWalletFromMnemonic and hdWalletFromXprv is proven harmless: for the
same master key both reach the same xpub and the same addresses.

vault.js: round trip (including non-ASCII and an empty password), wrong
password rejected as a rejected promise with no partial plaintext,
tampered ciphertext / auth tag / nonce / salt rejected, truncated and
spliced blobs rejected, missing fields rejected, fresh salt and nonce per
encryption, the documented { salt, nonce, ciphertext } shape, and no
trace of the plaintext or password anywhere in the serialized blob. The
production Argon2id parameters are not weakened; the tamper cases share
one encrypted fixture to stay inside script/test's 30-second budget.

One test is skipped: isValidXprv accepts an extended private key with a
one-character typo, because ethers skips base58 checksum verification for
the usual 82-byte payload. That is a defect to be filed separately, not
fixed here; the skipped test asserts the correct behaviour and names the
reason.

Suite: 211 passed, 1 skipped, 7.2s inside the container build.

TODO.md

@@ -44,6 +44,9 @@ undefined identifiers, which is how
 
 # Completed Steps
 
+- 2026-08-11: Known-answer test coverage for the crypto core — BIP-39/BIP-32
+  derivation in `wallet.js` and the Argon2id vault in `vault.js`
+  ([#159](https://git.eeqj.de/sneak/AutistMask/issues/159)).
 - 2026-08-11: `docs/README.md` rewritten against the code: no competitor names,
   all five network destinations documented, password/Settings/Add Wallet
   sections corrected ([#163](https://git.eeqj.de/sneak/AutistMask/issues/163)).