[feat](trx-rs): add --check-config to the server and client

Validating a config meant starting the daemon and reading the first error it
died on, fixing that, and repeating.  Add --check-config, which loads the
config through the real loader, reports every problem at once and exits 0/1:

    $ trx-server --check-config --config trx-rs.toml
    trx-rs.toml
      warning: unknown config key 'listen.prot' (did you mean 'listen.port'?)
      error: [general].log_level 'verbose' is invalid (expected one of: ...)
      error: [rig.access].baud must be > 0 for serial access
      error: [audio].frame_duration_ms must be one of: 3, 5, 10, 20, 40, 60
      error: [listen] and rig "default" [audio] would both bind 127.0.0.1:4530

Validation grows validate_all()/validate_resolved_all() alongside the existing
first-error entry points; validate() is now the first element of validate_all().
Sockets are built by one helper shared by startup and the check, so the two
cannot disagree about what --listen overrides.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SyX26FCpMQxiBoC7r5K1A7
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/src/main.rs

@@ -53,6 +53,9 @@ struct Cli {
     /// Treat unknown configuration keys as a fatal error
     #[arg(long = "strict-config")]
     strict_config: bool,
+    /// Validate the configuration and exit without starting anything
+    #[arg(long = "check-config")]
+    check_config: bool,
     /// Remote server URL (host:port)
     #[arg(short = 'u', long = "url")]
     url: Option<String>,
@@ -112,6 +115,59 @@ async fn main() -> DynResult<()> {
     Ok(())
 }
 
+/// `--check-config`: report everything wrong with the configuration and exit.
+///
+/// Unlike startup, this reports every problem it finds rather than stopping at
+/// the first, so a config can be fixed in one pass.  The file is checked as
+/// written, without CLI overrides.
+fn check_config(loaded: &trx_config::ConfigLoad<ClientConfig>) -> DynResult<()> {
+    match &loaded.path {
+        Some(path) => println!("{}", path.display()),
+        None => println!("(no config file found; checking built-in defaults)"),
+    }
+
+    let mut warnings: Vec<String> = loaded.unknown_keys.iter().map(|k| k.to_string()).collect();
+
+    let cfg = &loaded.config;
+    let remotes = cfg.resolved_remotes();
+    if remotes.is_empty() {
+        warnings.push(
+            "no remotes configured; --url will be required at startup (add [[remotes]] entries)"
+                .to_string(),
+        );
+    }
+
+    let mut errors = cfg.validate_all();
+    if !remotes.is_empty() {
+        errors.extend(cfg.validate_resolved_all(&remotes));
+    }
+
+    for w in &warnings {
+        println!("  warning: {}", w);
+    }
+    for e in &errors {
+        println!("  error: {}", e);
+    }
+
+    if errors.is_empty() {
+        println!(
+            "  OK: {} remote(s) configured: {}",
+            remotes.len(),
+            remotes
+                .iter()
+                .map(|r| r.name.as_str())
+                .collect::<Vec<_>>()
+                .join(", ")
+        );
+        if !warnings.is_empty() {
+            println!("  {} warning(s)", warnings.len());
+        }
+        Ok(())
+    } else {
+        Err(format!("{} error(s), {} warning(s)", errors.len(), warnings.len()).into())
+    }
+}
+
 /// Holds the state needed after async initialization completes.
 struct AppState {
     shutdown_tx: watch::Sender<bool>,
@@ -145,6 +201,16 @@ async fn async_init() -> DynResult<AppState> {
     };
     let config_path = loaded.path.clone();
 
+    if cli.check_config {
+        match check_config(&loaded) {
+            Ok(()) => std::process::exit(0),
+            Err(e) => {
+                eprintln!("{}", e);
+                std::process::exit(1);
+            }
+        }
+    }
+
     // Logging comes up before any config complaint so the warnings are visible.
     init_logging(loaded.config.general.log_level.as_deref());