refactor(daemon): AppState delegates to storage modules; dbus tests use tempdir

Lands plan tasks 2.15 (AppState refactor with StorageConfig + open()) and 2.16
(dbus.rs tests switched to tempdir-backed AppState; storage errors threaded
through D-Bus methods as Failed). Bundled because the AppState surface change
forces dbus.rs adjustments in the same commit.

- daemon/src/state.rs: AppState::open(StorageConfig) replaces with_fixtures().
  StorageConfig.from_env_or_defaults() reads AUTHFORGE_POLICY_DIR /
  _PENDING_DIR / _USERDB env vars (defaults: /etc/authforge/policy.d,
  /var/lib/authforge/pending, /var/lib/authforge/users.db). State delegates
  list/add/remove credentials to CredsPathResolver + CredentialsStore picked
  per-call from current Policy; pending and userdb operate independently.
- daemon/src/main.rs: opens state via env-driven config; reuses cfg.policy_dir
  for the watcher to keep one source of truth.
- daemon/src/dbus.rs: every write method maps StateError to fdo::Error::Failed.
  p2p_pair seeds 00-test.conf with [storage] backend = central pointing into
  the tempdir so credential writes don't try to touch /home/<user>/...
  (alice/bob/carol aren't real accounts in tests).
- Renamed: list_credentials_returns_fixture_for_alice ->
  list_credentials_after_enroll. Removed: with_fixtures().
- .gitignore: add .claude/ so leftover Phase 1 worktree state isn't committed.

Test count: 26/26 daemon tests green (was 17). Common: 13/13. Clippy + fmt clean.

daemon/src/storage/credentials.rs

@@ -7,10 +7,7 @@ pub(crate) enum CredsError {
     #[error("io: {0}")]
     Io(#[from] std::io::Error),
     #[error("malformed line for {user:?}: {reason}")]
-    Malformed {
-        user: String,
-        reason: &'static str,
-    },
+    Malformed { user: String, reason: &'static str },
 }
 
 /// One line of a `pam_u2f` keys file. Format per pam_u2f(8):