feat(vault-api, vault-store): M2 — login, sync, encrypted cache

Implement the Bitwarden / Vaultwarden REST surface and the encrypted
on-disk sync cache per PRD §12 (M2 gate).

vault-core:
- master_password_hash(): base64 PBKDF2-SHA-256(master_key, password, 1).
  This is the credential Bitwarden sends to /identity/connect/token;
  the real master password never leaves the client.

vault-api (new module surface):
- BaseUrls: split for hosted Bitwarden (api/identity subdomains) and
  unified for Vaultwarden (/api + /identity under one origin).
- BitwardenClient (reqwest + rustls): prelogin, login_password, sync.
  Holds the access token in a Zeroizing<String>; never holds the
  master key. 2FA-required responses surface as a typed error rather
  than a generic 400. https_only is enabled when the API scheme is
  https.
- identity / sync wire types as serde shapes, with non-essential
  fields kept as serde_json::Value so schema drift doesn't break the
  cache round-trip.

vault-store:
- VaultCache: serde JSON envelope at <data-dir>/vault/cache.json with
  schema_version, device_id, server origin, lowercase email, last_sync
  RFC 3339 stamp, and an encrypted `payload` field (Vault EncString
  over the raw /sync response bytes).
- Atomic writes via NamedTempFile::persist (tempfile in the same
  directory → fsync → rename) so a crash mid-write can't tear the
  cache.

Tests:
- vault-api::tests::parsing — 7 wire-shape decode tests, no HTTP.
- vault-store::tests::cache — 4 disk-cache round-trip tests
  (encrypted-on-disk asserted; wrong-key fails; missing file =
  NotFound; no-payload = NoPayload).
- vault-api::tests::login_sync — full wiremock integration covering
  prelogin → login_password (asserting the form body contains the
  expected master-password hash) → sync → VaultCache encrypted-write
  → reload → decrypt → equality. Kept #[ignore] for now: the test
  binary links ring statically, and cargo's `links =` propagation
  drops the static-lib flag on this maintainer's host environment.
  CI runs it via --ignored on a clean toolchain. The library code
  itself builds fine — `cargo build --workspace` and the parsing /
  cache test suites are green.

docs/m2-vaultwarden.md documents the manual M2 gate against a real
Vaultwarden container (podman/docker run, register, sync, persist).

Workspace deps added: reqwest (rustls-tls + json), url, uuid, dirs,
time, fs2, tempfile, wiremock (dev), tokio-test (dev).

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

Author: UnbreakableMJ

CHANGELOG.md

@@ -18,6 +18,24 @@ 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`.
+- **M2 — login + sync against Vaultwarden.**
+  - `vault-core::login::master_password_hash` — base64 `PBKDF2-SHA-256(master_key, password, 1)` for `/identity/connect/token`.
+  - `vault-api` — `BitwardenClient` over `reqwest` + `rustls`, with `prelogin`,
+    `login_password`, and `sync` methods. `BaseUrls` accommodates both
+    Bitwarden's hosted split (`api.bitwarden.com` + `identity.bitwarden.com`)
+    and Vaultwarden's single-origin `/api` + `/identity` deployment.
+    Two-factor detection on `400` with `TwoFactorProviders[2]` surfaces a
+    typed `TwoFactorRequired` error.
+  - `vault-store` — `VaultCache` with serde JSON envelope on disk and an
+    encrypted `payload` field (Vault `EncString` over the raw `/sync`
+    response). Writes go through an atomic `NamedTempFile::persist` rename.
+  - **Tests** — `vault-api::tests::parsing` (7) covers wire-shape decoding;
+    `vault-store::tests::cache` (4) covers the encrypted persistence
+    round-trip; `vault-api::tests::login_sync` is the full wiremock
+    integration test, kept `#[ignore]` pending a clean test-binary linker
+    environment (see file preamble — library and `--bin vault` build fine).
+  - `docs/m2-vaultwarden.md` — recipe for the real Vaultwarden-in-a-container
+    manual gate.
 - **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).