refactor(v2): extract bterminal-core crate with EventSink trait

Create Cargo workspace at v2/ level with members: src-tauri,
bterminal-core, bterminal-relay. Extract PtyManager and SidecarManager
into shared bterminal-core crate with EventSink trait for abstracting
event emission. TauriEventSink wraps AppHandle. src-tauri pty.rs and
sidecar.rs become thin re-exports. Move Cargo.lock to workspace root.
Add v2/target/ to .gitignore.

v2/bterminal-core/src/pty.rs

@@ -0,0 +1,173 @@
+use portable_pty::{native_pty_system, CommandBuilder, MasterPty, PtySize};
+use serde::{Deserialize, Serialize};
+use std::collections::HashMap;
+use std::io::{BufReader, Write};
+use std::sync::{Arc, Mutex};
+use std::thread;
+use uuid::Uuid;
+
+use crate::event::EventSink;
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct PtyOptions {
+    pub shell: Option<String>,
+    pub cwd: Option<String>,
+    pub args: Option<Vec<String>>,
+    pub cols: Option<u16>,
+    pub rows: Option<u16>,
+}
+
+struct PtyInstance {
+    master: Box<dyn MasterPty + Send>,
+    writer: Box<dyn Write + Send>,
+}
+
+pub struct PtyManager {
+    instances: Arc<Mutex<HashMap<String, PtyInstance>>>,
+    sink: Arc<dyn EventSink>,
+}
+
+impl PtyManager {
+    pub fn new(sink: Arc<dyn EventSink>) -> Self {
+        Self {
+            instances: Arc::new(Mutex::new(HashMap::new())),
+            sink,
+        }
+    }
+
+    pub fn spawn(&self, options: PtyOptions) -> Result<String, String> {
+        let pty_system = native_pty_system();
+        let cols = options.cols.unwrap_or(80);
+        let rows = options.rows.unwrap_or(24);
+
+        let pair = pty_system
+            .openpty(PtySize {
+                rows,
+                cols,
+                pixel_width: 0,
+                pixel_height: 0,
+            })
+            .map_err(|e| format!("Failed to open PTY: {e}"))?;
+
+        let shell = options.shell.unwrap_or_else(|| {
+            std::env::var("SHELL").unwrap_or_else(|_| "/bin/bash".to_string())
+        });
+
+        let mut cmd = CommandBuilder::new(&shell);
+        if let Some(args) = &options.args {
+            for arg in args {
+                cmd.arg(arg);
+            }
+        }
+        if let Some(cwd) = &options.cwd {
+            cmd.cwd(cwd);
+        }
+
+        let _child = pair
+            .slave
+            .spawn_command(cmd)
+            .map_err(|e| format!("Failed to spawn command: {e}"))?;
+
+        drop(pair.slave);
+
+        let id = Uuid::new_v4().to_string();
+        let reader = pair
+            .master
+            .try_clone_reader()
+            .map_err(|e| format!("Failed to clone PTY reader: {e}"))?;
+        let writer = pair
+            .master
+            .take_writer()
+            .map_err(|e| format!("Failed to take PTY writer: {e}"))?;
+
+        let event_id = id.clone();
+        let sink = self.sink.clone();
+        thread::spawn(move || {
+            let mut buf_reader = BufReader::with_capacity(4096, reader);
+            let mut buf = vec![0u8; 4096];
+            loop {
+                match std::io::Read::read(&mut buf_reader, &mut buf) {
+                    Ok(0) => {
+                        sink.emit(
+                            &format!("pty-exit-{event_id}"),
+                            serde_json::Value::Null,
+                        );
+                        break;
+                    }
+                    Ok(n) => {
+                        let data = String::from_utf8_lossy(&buf[..n]).to_string();
+                        sink.emit(
+                            &format!("pty-data-{event_id}"),
+                            serde_json::Value::String(data),
+                        );
+                    }
+                    Err(e) => {
+                        log::error!("PTY read error for {event_id}: {e}");
+                        sink.emit(
+                            &format!("pty-exit-{event_id}"),
+                            serde_json::Value::Null,
+                        );
+                        break;
+                    }
+                }
+            }
+        });
+
+        let instance = PtyInstance {
+            master: pair.master,
+            writer,
+        };
+        self.instances.lock().unwrap().insert(id.clone(), instance);
+
+        log::info!("Spawned PTY {id} ({shell})");
+        Ok(id)
+    }
+
+    pub fn write(&self, id: &str, data: &str) -> Result<(), String> {
+        let mut instances = self.instances.lock().unwrap();
+        let instance = instances
+            .get_mut(id)
+            .ok_or_else(|| format!("PTY {id} not found"))?;
+        instance
+            .writer
+            .write_all(data.as_bytes())
+            .map_err(|e| format!("PTY write error: {e}"))?;
+        instance
+            .writer
+            .flush()
+            .map_err(|e| format!("PTY flush error: {e}"))?;
+        Ok(())
+    }
+
+    pub fn resize(&self, id: &str, cols: u16, rows: u16) -> Result<(), String> {
+        let instances = self.instances.lock().unwrap();
+        let instance = instances
+            .get(id)
+            .ok_or_else(|| format!("PTY {id} not found"))?;
+        instance
+            .master
+            .resize(PtySize {
+                rows,
+                cols,
+                pixel_width: 0,
+                pixel_height: 0,
+            })
+            .map_err(|e| format!("PTY resize error: {e}"))?;
+        Ok(())
+    }
+
+    pub fn kill(&self, id: &str) -> Result<(), String> {
+        let mut instances = self.instances.lock().unwrap();
+        if instances.remove(id).is_some() {
+            log::info!("Killed PTY {id}");
+            Ok(())
+        } else {
+            Err(format!("PTY {id} not found"))
+        }
+    }
+
+    /// List active PTY session IDs.
+    pub fn list_sessions(&self) -> Vec<String> {
+        self.instances.lock().unwrap().keys().cloned().collect()
+    }
+}