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/src-tauri/src/event_sink.rs

@@ -0,0 +1,11 @@
+use bterminal_core::event::EventSink;
+use tauri::{AppHandle, Emitter};
+
+/// Bridges bterminal-core's EventSink trait to Tauri's event system.
+pub struct TauriEventSink(pub AppHandle);
+
+impl EventSink for TauriEventSink {
+    fn emit(&self, event: &str, payload: serde_json::Value) {
+        let _ = self.0.emit(event, &payload);
+    }
+}