cross-impl: prove Python/TS profile interop, fix two compatibility bugs

Fixtures show both implementations compute the same values. This adds the
missing half: proof they can read each other's on-disk profiles and
backups, using each side's real code paths.

scripts/cross_impl_check.py (12 checks, wired into CI):
  A. Python creates a profile -> TS opens the index, derives every secret
     identically, and unlocks it with the master password alone
  B. TS creates a profile -> Python decrypts parent_seed.enc and the index,
     recovers the same seed, derives the same secrets
  C. Portable backups round-trip both directions (Python verifies the
     checksum of a TS-written export)
  D. Both implementations reject a bad-checksum mnemonic

Two real compatibility bugs it caught, both fixed:

- TS-created profiles were unopenable by Python. Python only honors the
  recorded parent-seed KDF parameters when kdf.name is exactly "pbkdf2";
  TS wrote "pbkdf2-sha256" with an empty salt and 100k iterations, so
  Python fell back to its config defaults (200k, fingerprint salt) and
  derived a different key. TS now writes exactly what
  PasswordManager._build_seed_kdf_config does, and its own unlock path
  honors the recorded salt and iteration count.

- TS accepted invalid BIP-39 mnemonics. Seed derivation takes any string,
  so a typo'd phrase silently produced a *different* vault -- unopenable
  by Python and unrecoverable by the user. Mnemonics are now validated
  wherever they enter (profile creation and SEEDPASS_MNEMONIC), with an
  error that says why it matters.

Also: docs/typescript_cutover_plan.md records the gate list for promoting
TS to main and retiring Python to legacy/reference -- P0 gates 1-4 are
green, 5-7 (feature parity tail, packaging, migration/rollback) are not.
The Python tree stays in CI after cutover as the compatibility oracle.

Regression tests for both bugs. 151 core + 46 CLI tests green,
12/12 cross-implementation checks green.

Co-Authored-By: Claude Fable 5 <[email protected]>

js/packages/cli/src/program.ts

@@ -18,6 +18,7 @@ import {
   importBackup,
   exportBackup,
   generateFingerprint,
+  assertValidMnemonic,
   modifyEntry,
   archiveEntry,
   restoreEntry,
@@ -87,7 +88,12 @@ async function currentFingerprint(app: AppDir, opts: GlobalOpts): Promise<string
  */
 async function resolveMnemonic(app: AppDir, opts: GlobalOpts): Promise<string> {
   const env = process.env["SEEDPASS_MNEMONIC"];
-  if (env) return env;
+  if (env) {
+    // Catch a typo'd phrase at the boundary rather than deriving a silently
+    // different vault from it.
+    assertValidMnemonic(env, "SEEDPASS_MNEMONIC");
+    return env;
+  }
   // A process that authenticates with a scoped token has declared itself a
   // constrained principal: it must never escalate to owner access by
   // pulling the mnemonic from the unlocked agent.