feat(compositor): implement surface compositing and layer shell rendering

Replace the clear-colour-only stub in render_output with full
surface compositing via Space::render_elements_for_output.

Changes:
- drm_device.rs: add start_time: Instant to WeftDrmData for elapsed-
 time frame callbacks
- drm.rs: rewrite render_output rendering block
 - collect SpaceRenderElements from Space via render_elements_for_output
 which includes both mapped windows and wlr-layer-shell surfaces from
 layer_map_for_output (sorted by z-index, clipped to output geometry)
 - pass collected elements to DrmOutput::render_frame
 - fix send_frame timing from Duration::ZERO to start_time.elapsed()
 with 16ms throttle hint
 - add space.refresh() and popups.cleanup() after each frame
 Use explicit inner block to scope space+drm borrows so post-render
 bookkeeping can access state mutably

Cursor rendering deferred requires cursor theme loading or MemoryRenderBuffer
setup; tracked separately.

crates/weft-compositor/src/backend/drm.rs

@@ -5,7 +5,12 @@ pub fn run() -> anyhow::Result<()> {
 }
 
 #[cfg(target_os = "linux")]
-use std::{collections::HashMap, path::Path, sync::Arc, time::Duration};
+use std::{
+    collections::HashMap,
+    path::Path,
+    sync::Arc,
+    time::{Duration, Instant},
+};
 
 #[cfg(target_os = "linux")]
 use anyhow::Context;
@@ -71,6 +76,7 @@ const SUPPORTED_FORMATS: &[Fourcc] = &[
 const SUPPORTED_FORMATS_8BIT_ONLY: &[Fourcc] = &[Fourcc::Abgr8888, Fourcc::Argb8888];
 
 #[cfg(target_os = "linux")]
+#[allow(dead_code)]
 type WeftMultiRenderer<'a> = MultiRenderer<
     'a,
     'a,
@@ -239,6 +245,7 @@ pub fn run() -> anyhow::Result<()> {
         devices: HashMap::new(),
         keyboards: Vec::new(),
         display_handle,
+        start_time: Instant::now(),
     });
 
     let existing: Vec<(DrmNode, std::path::PathBuf)> = state
@@ -581,6 +588,13 @@ fn render_output(state: &mut WeftCompositorState, node: DrmNode, crtc: crtc::Han
             .unwrap_or(d.primary_gpu)
     };
 
+    let elapsed = state
+        .drm
+        .as_ref()
+        .map(|d| d.start_time.elapsed())
+        .unwrap_or_default();
+
+    {
         let WeftCompositorState {
             ref mut drm,
             ref space,
@@ -615,12 +629,13 @@ fn render_output(state: &mut WeftCompositorState, node: DrmNode, crtc: crtc::Han
             }
         };
 
-    // Wave 2: clear-colour-only frame to establish the VBlank pipeline.
+        let elements = space
-    // Surface compositing is added in Wave 3.
+            .render_elements_for_output(&mut renderer, &output, 1.0)
-    let elements: &[WaylandSurfaceRenderElement<WeftMultiRenderer<'_>>] = &[];
+            .unwrap_or_default();
+
         match surface.drm_output.render_frame(
             &mut renderer,
-        elements,
+            &elements,
             [0.08_f32, 0.08, 0.08, 1.0],
             FrameFlags::DEFAULT,
         ) {
@@ -632,12 +647,14 @@ fn render_output(state: &mut WeftCompositorState, node: DrmNode, crtc: crtc::Han
             Ok(_) => {}
             Err(e) => tracing::warn!(?e, "render_frame failed"),
         }
+    }
 
-    space.elements().for_each(|window| {
+    state.space.elements().for_each(|window| {
-        window.send_frame(&output, Duration::ZERO, Some(Duration::ZERO), |_, _| {
+        window.send_frame(&output, elapsed, Some(Duration::from_millis(16)), |_, _| {
             Some(output.clone())
         });
     });
-
+    state.space.refresh();
+    state.popups.cleanup();
     let _ = state.display_handle.flush_clients();
 }

crates/weft-compositor/src/backend/drm_device.rs

@@ -1,4 +1,4 @@
-use std::collections::HashMap;
+use std::{collections::HashMap, time::Instant};
 
 use smithay::{
     backend::{
@@ -55,4 +55,5 @@ pub struct WeftDrmData {
     pub devices: HashMap<DrmNode, WeftDrmDevice>,
     pub keyboards: Vec<smithay::reexports::input::Device>,
     pub display_handle: DisplayHandle,
+    pub start_time: Instant,
 }