Add gamma control and night light (#38)

Add configurable night light, smooth gamma transitions, and shell controls. Update documentation and panel tests, and fix portal test startup synchronization.

shell/ferese-shell/src/status/night_light.rs

@@ -0,0 +1,72 @@
+//! Compositor status and config persistence for night light, off the UI thread.
+use std::sync::{Arc, mpsc};
+use std::time::Duration;
+
+use ferese_ipc::night_light::Status;
+use serde_json::json;
+
+use super::{PollState, run, wake_status};
+
+pub(super) fn snapshot() -> Option<Status> {
+    crate::compositor_ipc::call("night-light")
+        .ok()
+        .and_then(|value| serde_json::from_value(value).ok())
+}
+
+// Only a changed compositor status wakes the other adapters. Dropping the sender
+// cancels the wait; an in-flight IPC request already has a bounded timeout.
+pub(super) struct Monitor {
+    _stop: mpsc::Sender<()>,
+}
+
+impl Monitor {
+    pub(super) fn start(wake: Arc<PollState>) -> std::io::Result<Self> {
+        let (stop, cancelled) = mpsc::channel();
+        std::thread::Builder::new()
+            .name("ferese-night-light-status".into())
+            .spawn(move || {
+                let mut previous = None;
+                loop {
+                    let current = snapshot();
+                    if current != previous {
+                        previous = current;
+                        wake_status(&wake);
+                    }
+                    match cancelled.recv_timeout(Duration::from_secs(5)) {
+                        Err(mpsc::RecvTimeoutError::Timeout) => {}
+                        _ => break,
+                    }
+                }
+            })?;
+        Ok(Self { _stop: stop })
+    }
+}
+
+pub(super) enum Change {
+    Enabled(bool),
+    Temperature(u16),
+}
+
+impl Change {
+    pub(super) fn save(self) -> Result<(), String> {
+        let path = ferese_config::config_path().ok_or("No configuration directory")?;
+        let file = ferese_config::file::ConfigFile::open(&path)?;
+        let mut document = ferese_config::Document::parse(file.source()).map_err(|e| e.to_string())?;
+        let (field, value) = match self {
+            Self::Enabled(value) => ("night_light.enabled", json!(value)),
+            Self::Temperature(value) => ("night_light.temperature", json!(value)),
+        };
+        document.set(field, value).map_err(|e| e.to_string())?;
+        let settings: ferese_config::night_light::Settings =
+            serde_json::from_value(document.value()["night_light"].clone()).map_err(|e| e.to_string())?;
+        settings.validate()?;
+        file.replace(&document.to_string(), None, |staged| {
+            run(
+                "ferese",
+                &["--check-config", staged.to_str().ok_or("Invalid configuration path")?],
+            )
+            .map(|_| ())
+        })?;
+        crate::compositor_ipc::call("reload-config").map(|_| ())
+    }
+}