feat: phase 2 — handshake, message writers, and server message dispatch

handshake.rs:
- Config, PixelFormat, ServerInit, Session types
- connect() walks all 11 handshake steps per aw.g() line 226
- Auth error mapping from aw.a(int) line 350 (7 error codes)

msg.rs — client-to-server writers:
- SetEncodings (type 2), FramebufferUpdateRequest (type 3)
- KeyEvent (type 4), PointerEvent (type 5, 8 bytes)
- PingResponse (type 149), BandwidthMarker (type 151)

msg.rs — server message dispatch:
- ServerMsg enum covering all 15 message types
- Readers for ping, bandwidth probe, ack, debug string,
  RFB command, server cut text, server name update,
  layout/locale, RDP event, FB update header

examples/handshake.rs: connects and prints session info.

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

crates/ericrfb/examples/handshake.rs

@@ -0,0 +1,47 @@
+use std::env;
+
+use ericrfb::handshake::{Config, connect};
+
+fn main() {
+    let args: Vec<String> = env::args().collect();
+
+    let host = args
+        .iter()
+        .position(|a| a == "--host")
+        .and_then(|i| args.get(i + 1))
+        .expect("usage: --host <ip> --applet-id <token> [--port <port>]");
+
+    let applet_id = args
+        .iter()
+        .position(|a| a == "--applet-id")
+        .and_then(|i| args.get(i + 1))
+        .expect("usage: --host <ip> --applet-id <token> [--port <port>]");
+
+    let port: u16 = args
+        .iter()
+        .position(|a| a == "--port")
+        .and_then(|i| args.get(i + 1))
+        .and_then(|s| s.parse().ok())
+        .unwrap_or(443);
+
+    let cfg = Config::new(host, port, applet_id);
+    println!("Connecting to {}:{}...", cfg.host, cfg.port);
+
+    match connect(&cfg) {
+        Ok(session) => {
+            println!(
+                "Connected: name={:?}, {}x{}, version={}.{}, format={}",
+                session.server_name,
+                session.width(),
+                session.height(),
+                session.server_version.0,
+                session.server_version.1,
+                session.pixel_format.label,
+            );
+        }
+        Err(e) => {
+            eprintln!("Handshake failed: {e}");
+            std::process::exit(1);
+        }
+    }
+}