feat(vault-core): M1 — parse and decrypt Bitwarden encrypted exports

Implement the offline-decrypt path for password-protected Bitwarden
.json exports per PRD §12 (M1 gate).

New modules:
- enc_string: type-2 EncString parser/encoder (AES-256-CBC + HMAC-
  SHA-256, Encrypt-then-MAC, constant-time MAC verify via subtle).
  Legacy unauthenticated types 0/1 are explicitly rejected.
- kdf: PBKDF2-SHA-256 and Argon2id master-key derivation, plus the
  HKDF-SHA-256 stretch into a 64-byte (enc, mac) pair using the
  Bitwarden info labels ("enc" / "mac").
- export: serde envelope for the encrypted .json export and a
  decrypt() that validates the password via encKeyValidation_DO_NOT_EDIT
  before touching the data payload.
- error: thiserror-backed Error / Result.

Tests (9, all green): EncString round-trip, serialize/parse,
tampering detection (MAC catches a single-bit flip), legacy-type
rejection, end-to-end PBKDF2 export decrypt, end-to-end Argon2id
export decrypt, wrong-password rejection, envelope-shape rejection,
HKDF stretch sanity.

Dependencies added at workspace level: aes, cbc, cipher, hmac,
sha2, pbkdf2, argon2, hkdf, subtle, base64, hex, getrandom, rand.
All RustCrypto, MIT/Apache-2.0 — GPL-3.0-or-later compatible.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: UnbreakableMJ

CHANGELOG.md

@@ -18,3 +18,16 @@ range may break in any release.
   `LICENSE`, `CREDITS.md`, `CHANGELOG.md`.
 - `PRD.md` — full product requirements document.
 - CI configuration: `fmt`, `clippy -D warnings`, `cargo audit`, `cargo deny`.
+- **M1 — offline export decrypt.** `vault-core` now ships:
+  - `EncString` (Bitwarden type 2: AES-256-CBC + HMAC-SHA-256, Encrypt-then-MAC
+    with constant-time verification; legacy types 0/1 explicitly rejected).
+  - `kdf::{KdfType, KdfParams}` and `derive_master_key` covering
+    PBKDF2-SHA-256 and Argon2id (with Bitwarden's SHA-256 salt preprocessing).
+  - `stretch_master_key` — HKDF-SHA-256 expansion of a 32-byte master key
+    into a 64-byte `(enc, mac)` pair using the official `info="enc"` /
+    `info="mac"` labels.
+  - `EncryptedExport` — parser and decryptor for password-protected Bitwarden
+    `.json` exports, validating the password against `encKeyValidation_DO_NOT_EDIT`
+    before touching the data payload.
+  - 9 integration tests covering round-trip, tampering detection, both KDFs,
+    wrong-password rejection, and envelope-shape validation.