refactor(state): split 1598-LOC state.rs into directory + extract 13 Dispatch impls

Step 4a + 4b combined.

- src/state.rs (1594 LOC) -> src/state/mod.rs (struct + inherent methods +
  types + helpers; 999 LOC) + src/state/dispatch/ (13 Dispatch impls across
  6 files: registry.rs / wl_output.rs / dmabuf.rs / screencopy.rs /
  output_mgr.rs / buffer.rs).

Per Oracle audit: orphan rule permits Dispatch impls in submodules because
Dispatch is a foreign trait on local type State<S>. All State fields the
impls touch are already pub/pub(crate) — no visibility widening needed.

Verification (all green):
- cargo build / cargo build --release
- cargo test (79 lib + 3 integration = 82 pass, 1 ignored — unchanged)
- cargo clippy --all-targets -- -D warnings
- cargo fmt --check
- cargo check --bin vaapi_import_bench --bin sw_encode_bench

src/state/dispatch/output_mgr.rs

@@ -0,0 +1,109 @@
+use wayland_client::{event_created_child, Dispatch, Proxy, QueueHandle};
+use wayland_protocols::xdg::xdg_output::zv1::client::zxdg_output_manager_v1::ZxdgOutputManagerV1;
+use wayland_protocols_wlr::output_management::v1::client::zwlr_output_head_v1::{
+    self, Event as WlrHeadEvent, ZwlrOutputHeadV1,
+};
+use wayland_protocols_wlr::output_management::v1::client::zwlr_output_manager_v1::{
+    self, Event as WlrOutputManagerEvent, ZwlrOutputManagerV1,
+};
+use wayland_protocols_wlr::output_management::v1::client::zwlr_output_mode_v1::ZwlrOutputModeV1;
+
+use crate::state::{CaptureSource, EncConstructionStage, State, WlrHeadInfo};
+
+impl<S: CaptureSource> Dispatch<ZxdgOutputManagerV1, ()> for State<S> {
+    fn event(
+        _state: &mut Self,
+        _proxy: &ZxdgOutputManagerV1,
+        _event: <ZxdgOutputManagerV1 as Proxy>::Event,
+        _data: &(),
+        _conn: &wayland_client::Connection,
+        _qhandle: &QueueHandle<State<S>>,
+    ) {
+    }
+}
+
+impl<S: CaptureSource> Dispatch<ZwlrOutputManagerV1, ()> for State<S> {
+    fn event(
+        state: &mut Self,
+        _proxy: &ZwlrOutputManagerV1,
+        event: <ZwlrOutputManagerV1 as Proxy>::Event,
+        _data: &(),
+        _conn: &wayland_client::Connection,
+        _qhandle: &QueueHandle<State<S>>,
+    ) {
+        match event {
+            WlrOutputManagerEvent::Head { head } => {
+                let _head: ZwlrOutputHeadV1 = head;
+                tracing::debug!("wlr output head advertised");
+            }
+            WlrOutputManagerEvent::Done { .. } => {
+                if let EncConstructionStage::ProbingOutputs {
+                    wlr_manager_done,
+                    outputs,
+                    ..
+                } = &mut state.stage
+                {
+                    *wlr_manager_done = true;
+                    let count = outputs.len();
+                    for idx in 0..count {
+                        state.try_finalize_output(idx);
+                    }
+                }
+            }
+            WlrOutputManagerEvent::Finished => {
+                tracing::warn!("zwlr_output_manager_v1::Finished received during probing");
+            }
+            _ => {}
+        }
+    }
+
+    event_created_child!(State<S>, ZwlrOutputManagerV1, [
+        zwlr_output_manager_v1::EVT_HEAD_OPCODE => (ZwlrOutputHeadV1, ()),
+    ]);
+}
+
+impl<S: CaptureSource> Dispatch<ZwlrOutputHeadV1, ()> for State<S> {
+    fn event(
+        state: &mut Self,
+        proxy: &ZwlrOutputHeadV1,
+        event: <ZwlrOutputHeadV1 as Proxy>::Event,
+        _data: &(),
+        _conn: &wayland_client::Connection,
+        _qhandle: &QueueHandle<State<S>>,
+    ) {
+        match event {
+            WlrHeadEvent::Name { name } => {
+                if let EncConstructionStage::ProbingOutputs {
+                    wlr_heads,
+                    wlr_head_proxy_to_name,
+                    ..
+                } = &mut state.stage
+                {
+                    wlr_heads.entry(name.clone()).or_insert(WlrHeadInfo {});
+                    wlr_head_proxy_to_name.insert(proxy.id(), name);
+                }
+            }
+            WlrHeadEvent::Position { .. } => {}
+            WlrHeadEvent::Finished => {
+                tracing::debug!("zwlr_output_head_v1::Finished received");
+            }
+            _ => {}
+        }
+    }
+
+    event_created_child!(State<S>, ZwlrOutputHeadV1, [
+        zwlr_output_head_v1::EVT_MODE_OPCODE => (ZwlrOutputModeV1, ()),
+    ]);
+}
+
+impl<S: CaptureSource> Dispatch<ZwlrOutputModeV1, ()> for State<S> {
+    fn event(
+        _state: &mut Self,
+        _proxy: &ZwlrOutputModeV1,
+        _event: <ZwlrOutputModeV1 as Proxy>::Event,
+        _data: &(),
+        _conn: &wayland_client::Connection,
+        _qhandle: &QueueHandle<State<S>>,
+    ) {
+    }
+}