feat(protocol): A.1 — fix hostname resolution on Android and iOS

Resolves the Beta-blocking issue surfaced during Android v0.2.0-beta.1
verification: hostnames could not be used, only literal IPs.

Root cause:
  tsclientlib's built-in resolver uses hickory-resolver, which reads
  /etc/resolv.conf. That file does not exist on Android or iOS, so
  any connect by hostname exited the connection task before
  signalling ready and surfaced the cryptic error
    BridgeError.connection(field0: protocol backend:
      connection task exited before signalling ready)

Fix:
  crates/chanora_protocol/src/resolver.rs (new):
    Resolves hostnames via tokio::net::lookup_host, which uses the
    platform's getaddrinfo. Works on every platform Chanora targets.
    Tiny in-process positive-result cache (5 min TTL) keeps
    reconnects cheap. IPv4 sorted ahead of IPv6 in the returned list
    to favour the more reliable path on dual-stack networks.

  crates/chanora_protocol/src/adapter.rs:
    connection_task now resolves the hostname itself and passes the
    resulting SocketAddr (not the hostname String) to
    tsclientlib::Connection::build. tsclientlib's ServerAddress enum
    accepts SocketAddr via its From impl, so the upstream resolver
    is skipped entirely.

  crates/chanora_protocol/src/lib.rs:
    New typed error arm ProtocolError::DnsFailed { host, reason }
    so the UI can distinguish 'server not found' from 'server
    refused our packets'.

  crates/chanora_bridge/src/lib.rs:
    Matching BridgeError::DnsFailed { host, reason } DTO surfaced
    to Dart, with explicit From<CoreError::Protocol(DnsFailed)>
    mapping so the UI gets the structured fields rather than a
    stringified mess.

Tests added (crates/chanora_protocol/src/resolver.rs::tests):
  - rejects_empty
  - literal_ipv4_short_circuits
  - literal_ipv4_default_port_path
  - unresolvable_returns_dns_failed
  - resolves_known_hostname  (#[ignore], --ignored to run; hits net)

Empirical verification (2026-05-14):
  Workspace: cargo check + cargo test --workspace clean.
  Live resolver test: cn.teamspeak.app → 175.178.125.23:9987 (passes).
  cargo test -p chanora_core --test alpha_smoke -- --ignored:
    server='Vigorous Pro' channels=42 clients=20 (passes by hostname).
  flutter test: alpha_e2e_test + beta_e2e_test both green.
  Physical Moto G Stylus 5G (Android 14 arm64-v8a):
    APK rebuilt (48.9 MB). adb install + launch.
    Connect form left at default 'cn.teamspeak.app'.
    logcat shows:
      chanora_protocol: dns resolved input=cn.teamspeak.app
                                    resolved=175.178.125.23:9987
      tsclientlib: starting connection to 175.178.125.23:9987
      tsproto::resend: Connecting → Connected
      chanora_protocol: initial state snapshot received
    UI shows 'Connected to Vigorous Pro' / '42 channels • 20 online'.

This is the first item in Category A (post-Beta polish bundle).
Pause point: review before A.6 (full reconnect).

crates/chanora_bridge/src/lib.rs

@@ -47,6 +47,15 @@ pub enum BridgeError {
     /// The caller submitted a malformed command DTO.
     #[error("invalid command: {0}")]
     InvalidCommand(String),
+    /// Hostname resolution failed. Distinct from `Connection` so the
+    /// UI can show a meaningful "Server not found" message.
+    #[error("dns: could not resolve '{host}': {reason}")]
+    DnsFailed {
+        /// The hostname (or `host:port`) the caller submitted.
+        host: String,
+        /// Reason from the platform resolver.
+        reason: String,
+    },
     /// Connection layer failure (typed-mapped from CoreError).
     #[error("connection: {0}")]
     Connection(String),
@@ -72,6 +81,10 @@ impl From<chanora_core::CoreError> for BridgeError {
             chanora_core::CoreError::AudioNotStarted => {
                 BridgeError::InvalidCommand("audio not started".to_string())
             }
+            chanora_core::CoreError::Protocol(chanora_protocol::ProtocolError::DnsFailed {
+                host,
+                reason,
+            }) => BridgeError::DnsFailed { host, reason },
             chanora_core::CoreError::Protocol(p) => BridgeError::Connection(format!("{p}")),
             chanora_core::CoreError::Audio(a) => BridgeError::Connection(format!("audio: {a}")),
             other => BridgeError::Unmapped(format!("{other}")),