Add OpenWrt ipk packaging + split client/server binaries

OpenWrt package (deploy/openwrt/):
- build-ipk.sh: creates .ipk from pre-built binary (no SDK needed)
- Makefile: for OpenWrt SDK integration
- ProCD init script with UCI config
- Supports all architectures (x86_64, aarch64, mipsel, mips)

Split binaries for embedded (src/bin/):
- btest-client: client-only, no server/syslog/csv
- btest-server: server-only, no client
- release-small profile: opt-level=z + panic=abort

Sizes (compressed .tar.gz):
  Full btest:    ~1 MB
  btest-client:  ~500 KB (release-small)
  btest-server:  ~550 KB (release-small)

Install on OpenWrt:
  opkg install btest-rs_0.6.0-1_x86_64.ipk

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/bin/server_only.rs

@@ -0,0 +1,62 @@
+//! btest-server: minimal bandwidth test server for embedded/OpenWrt systems.
+//!
+//! Stripped-down server that accepts MikroTik client connections.
+//! No client mode, no syslog, no CSV, smaller binary footprint.
+//!
+//! Build: cargo build --profile release-small --bin btest-server
+
+use clap::Parser;
+
+#[derive(Parser)]
+#[command(name = "btest-server", about = "MikroTik Bandwidth Test server", version)]
+struct Cli {
+    /// Port
+    #[arg(short = 'P', long = "port", default_value_t = 2000)]
+    port: u16,
+
+    /// IPv4 listen address
+    #[arg(long = "listen", default_value = "0.0.0.0")]
+    listen_addr: String,
+
+    /// Username
+    #[arg(short = 'a', long = "authuser")]
+    auth_user: Option<String>,
+
+    /// Password
+    #[arg(short = 'p', long = "authpass")]
+    auth_pass: Option<String>,
+
+    /// Use EC-SRP5 authentication
+    #[arg(long = "ecsrp5")]
+    ecsrp5: bool,
+
+    /// Verbose
+    #[arg(short = 'v', long = "verbose", action = clap::ArgAction::Count)]
+    verbose: u8,
+}
+
+#[tokio::main]
+async fn main() -> anyhow::Result<()> {
+    let cli = Cli::parse();
+
+    let filter = match cli.verbose {
+        0 => "info",
+        1 => "debug",
+        _ => "trace",
+    };
+    tracing_subscriber::fmt()
+        .with_env_filter(
+            tracing_subscriber::EnvFilter::try_from_default_env()
+                .unwrap_or_else(|_| tracing_subscriber::EnvFilter::new(filter)),
+        )
+        .with_target(false)
+        .init();
+
+    btest_rs::cpu::start_sampler();
+
+    let v4 = if cli.listen_addr.eq_ignore_ascii_case("none") { None } else { Some(cli.listen_addr) };
+
+    tracing::info!("btest-server starting on port {}", cli.port);
+    btest_rs::server::run_server(cli.port, cli.auth_user, cli.auth_pass, cli.ecsrp5, v4, None).await?;
+    Ok(())
+}