feat(storage): A.2 — persist TS3 identity across app restarts

A fresh `Identity::create()` was generated on every connect, which
meant the server saw a different client UID each time. Long-lived
features (bookmarks, server-side bans, group membership) depend on a
stable UID — restoring that now via a minimal directory-backed
identity file.

* `chanora_storage::IdentityFileStore` reads / writes a single
  `identity.tskey` file under a caller-supplied directory. On Unix
  the file is created with `O_CREAT | O_TRUNC | mode 0600`; on
  non-Unix targets the platform sandbox does the access control.
  Writes are atomic (temp file + `fsync` + `rename`) so a crash
  mid-write cannot leave a half-written identity on disk. Empty
  files are treated as "no identity" rather than as an error.
* `chanora_protocol::ProtocolClient::generate_identity()` exposes
  the `counterVbase64key` serialisation used by tsclientlib's
  `Identity::new_from_str`, so the core layer can mint an identity
  and store it before dialling.
* `chanora_core::ChanoraSession::init_storage(dir)` wires the
  store. `connect()` then resolves the identity in this order:
  (1) `cfg.identity` if explicitly supplied; (2) persisted value if
  any; (3) generate-and-persist a fresh one.
* `chanora_bridge::api::init_storage(dir: String)` is the
  Flutter-facing entrypoint; the matching Dart side resolves
  `path_provider`'s `getApplicationSupportDirectory()` and calls
  it once on app start.
* `BridgeError` now maps `CoreError::Storage`.

Beta caveat (RISK-PoC-002 / SS-RISK-FALLBACK): the identity is not
encrypted at rest. The v0.4 storage rework lands proper Secret
Service + Android Keystore + iOS Keychain backends. Documented
under `IdentityFileStore`'s doc comment.

Live-verified on Moto G Stylus 5G: first connect generated +
persisted the identity (visible in the redacted diagnostic export
as "generated + persisted fresh identity"); disconnect + reconnect
in the same session logged "reusing persisted identity" and dialled
with the same UID.

crates/chanora_bridge/src/frb_generated.rs

@@ -38,7 +38,7 @@ flutter_rust_bridge::frb_generated_boilerplate!(
     default_rust_auto_opaque = RustAutoOpaqueMoi,
 );
 pub(crate) const FLUTTER_RUST_BRIDGE_CODEGEN_VERSION: &str = "2.12.0";
-pub(crate) const FLUTTER_RUST_BRIDGE_CODEGEN_CONTENT_HASH: i32 = -1212711005;
+pub(crate) const FLUTTER_RUST_BRIDGE_CODEGEN_CONTENT_HASH: i32 = 1702138901;
 
 // Section: executor
 
@@ -223,6 +223,42 @@ fn wire__crate__api__events_stream_impl(
         },
     )
 }
+fn wire__crate__api__init_storage_impl(
+    port_: flutter_rust_bridge::for_generated::MessagePort,
+    ptr_: flutter_rust_bridge::for_generated::PlatformGeneralizedUint8ListPtr,
+    rust_vec_len_: i32,
+    data_len_: i32,
+) {
+    FLUTTER_RUST_BRIDGE_HANDLER.wrap_async::<flutter_rust_bridge::for_generated::SseCodec, _, _, _>(
+        flutter_rust_bridge::for_generated::TaskInfo {
+            debug_name: "init_storage",
+            port: Some(port_),
+            mode: flutter_rust_bridge::for_generated::FfiCallMode::Normal,
+        },
+        move || {
+            let message = unsafe {
+                flutter_rust_bridge::for_generated::Dart2RustMessageSse::from_wire(
+                    ptr_,
+                    rust_vec_len_,
+                    data_len_,
+                )
+            };
+            let mut deserializer =
+                flutter_rust_bridge::for_generated::SseDeserializer::new(message);
+            let api_dir = <String>::sse_decode(&mut deserializer);
+            deserializer.end();
+            move |context| async move {
+                transform_result_sse::<_, crate::BridgeError>(
+                    (move || async move {
+                        let output_ok = crate::api::init_storage(api_dir).await?;
+                        Ok(output_ok)
+                    })()
+                    .await,
+                )
+            }
+        },
+    )
+}
 fn wire__crate__api__is_connected_impl(
     port_: flutter_rust_bridge::for_generated::MessagePort,
     ptr_: flutter_rust_bridge::for_generated::PlatformGeneralizedUint8ListPtr,
@@ -677,10 +713,11 @@ fn pde_ffi_dispatcher_primary_impl(
         3 => wire__crate__api__connect_impl(port, ptr, rust_vec_len, data_len),
         4 => wire__crate__api__disconnect_impl(port, ptr, rust_vec_len, data_len),
         5 => wire__crate__api__events_stream_impl(port, ptr, rust_vec_len, data_len),
-        6 => wire__crate__api__is_connected_impl(port, ptr, rust_vec_len, data_len),
-        8 => wire__crate__api__set_ptt_impl(port, ptr, rust_vec_len, data_len),
-        9 => wire__crate__api__snapshot_impl(port, ptr, rust_vec_len, data_len),
-        10 => wire__crate__api__start_audio_impl(port, ptr, rust_vec_len, data_len),
+        6 => wire__crate__api__init_storage_impl(port, ptr, rust_vec_len, data_len),
+        7 => wire__crate__api__is_connected_impl(port, ptr, rust_vec_len, data_len),
+        9 => wire__crate__api__set_ptt_impl(port, ptr, rust_vec_len, data_len),
+        10 => wire__crate__api__snapshot_impl(port, ptr, rust_vec_len, data_len),
+        11 => wire__crate__api__start_audio_impl(port, ptr, rust_vec_len, data_len),
         _ => unreachable!(),
     }
 }
@@ -693,7 +730,7 @@ fn pde_ffi_dispatcher_sync_impl(
 ) -> flutter_rust_bridge::for_generated::WireSyncRust2DartSse {
     // Codec=Pde (Serialization + dispatch), see doc to use other codecs
     match func_id {
-        7 => wire__crate__api__set_network_state_impl(ptr, rust_vec_len, data_len),
+        8 => wire__crate__api__set_network_state_impl(ptr, rust_vec_len, data_len),
         _ => unreachable!(),
     }
 }