chore: scaffold tvctl foundation

Set up the Rust crate, baseline module layout, and project docs so the
repository matches the design bundle and builds cleanly as a starting point.

src/adapters/roku/mod.rs

@@ -0,0 +1,42 @@
+use super::{AppInfo, Device, DeviceInfo, DeviceState, Result, TvAdapter, TvError, TvKey};
+
+/// The Roku ECP adapter placeholder for the foundation milestone.
+#[derive(Debug, Clone, Default)]
+pub struct RokuAdapter;
+
+impl RokuAdapter {
+    /// Create a new Roku adapter instance.
+    pub fn new() -> Self {
+        Self
+    }
+}
+
+impl TvAdapter for RokuAdapter {
+    async fn discover(&self) -> Result<Vec<DeviceInfo>> {
+        Err(TvError::NotSupported("discover"))
+    }
+
+    async fn state(&self, _device: &Device) -> Result<DeviceState> {
+        Err(TvError::NotSupported("state"))
+    }
+
+    async fn launch(&self, _device: &Device, _app: &str) -> Result<()> {
+        Err(TvError::NotSupported("launch"))
+    }
+
+    async fn stop_app(&self, _device: &Device) -> Result<()> {
+        Err(TvError::NotSupported("stop_app"))
+    }
+
+    async fn key(&self, _device: &Device, _key: TvKey) -> Result<()> {
+        Err(TvError::NotSupported("key"))
+    }
+
+    async fn sequence(&self, _device: &Device, _keys: Vec<TvKey>) -> Result<()> {
+        Err(TvError::NotSupported("sequence"))
+    }
+
+    async fn list_apps(&self, _device: &Device) -> Result<Vec<AppInfo>> {
+        Err(TvError::NotSupported("list_apps"))
+    }
+}