feat: phase 3 — framebuffer, raw decoder, session pump, snapshot

framebuffer.rs:
- Framebuffer struct (8bpp RGB332, row-major)
- apply_raw() blit, copy_rect() with overlap-safe logic
- to_rgba() via compile-time RGB332 LUT

session.rs:
- ActiveSession: connect + SetEncodings + initial FBUpdateRequest
- Full server message dispatch loop (all 15 message types)
- Raw (encoding 0) and CopyRect (encoding 1) decoders
- Ping response, bandwidth probe bookends, mode change resize

examples/snapshot.rs:
- Connects, waits for first FramebufferUpdate, saves PNG
- Tested against real OmniView at 10.3.0.130:443
- Successfully captured 640x480 frame

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

crates/ericrfb/examples/snapshot.rs

@@ -0,0 +1,88 @@
+use std::env;
+use std::fs::File;
+use std::io::BufWriter;
+use std::path::Path;
+
+use ericrfb::handshake::Config;
+use ericrfb::session::{ActiveSession, Event};
+
+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>] [--output <file.png>]");
+
+    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>] [--output <file.png>]");
+
+    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 output = args
+        .iter()
+        .position(|a| a == "--output")
+        .and_then(|i| args.get(i + 1).map(|s| s.as_str()))
+        .unwrap_or("frame.png");
+
+    let cfg = Config::new(host, port, applet_id);
+    println!("Connecting to {}:{}...", cfg.host, cfg.port);
+
+    // Only request Raw encoding (0) for Phase 3
+    let mut session = ActiveSession::connect(&cfg, &[0]).expect("connect failed");
+    println!(
+        "Connected: {}x{}, waiting for first frame...",
+        session.framebuffer.width, session.framebuffer.height
+    );
+
+    // Process messages until we get a FramebufferDirty event
+    loop {
+        match session.process_one() {
+            Ok(Some(Event::FramebufferDirty)) => {
+                println!("Got framebuffer update, saving to {output}");
+                break;
+            }
+            Ok(Some(Event::Resize { width, height })) => {
+                println!("Resized to {width}x{height}");
+            }
+            Ok(Some(Event::Debug(s))) => {
+                eprintln!("[debug] {s}");
+            }
+            Ok(Some(Event::RfbCommand(k, v))) => {
+                eprintln!("[rfb-cmd] {k}={v}");
+            }
+            Ok(Some(_)) => {}
+            Ok(None) => {}
+            Err(e) => {
+                eprintln!("Error: {e}");
+                std::process::exit(1);
+            }
+        }
+    }
+
+    // Write PNG
+    let rgba = session.framebuffer.to_rgba();
+    let w = session.framebuffer.width as u32;
+    let h = session.framebuffer.height as u32;
+
+    let path = Path::new(output);
+    let file = File::create(path).expect("cannot create output file");
+    let bw = BufWriter::new(file);
+
+    let mut encoder = png::Encoder::new(bw, w, h);
+    encoder.set_color(png::ColorType::Rgba);
+    encoder.set_depth(png::BitDepth::Eight);
+    let mut writer = encoder.write_header().expect("png header failed");
+    writer.write_image_data(&rgba).expect("png write failed");
+
+    println!("Saved {w}x{h} frame to {}", path.display());
+}