refactor: clear remaining clippy dead-code and cast warnings

Brings `cargo clippy --release --all-targets` and `cargo build --release`
to zero warnings. Three categories:

Truly dead code (deleted):
- OutputInfo.physical_size / .logical_position — copied from PartialOutputInfo
  at construction but never read on OutputInfo; PartialOutputInfo still uses
  them as probe-completion gates
- EncConstructionStage::Streaming.output_info — stored at ->Streaming
  transition, all 9 match arms discard via `..` or `output_info: _`
- State.starting_timestamp — vestigial Phase 1 stub; PTS normalization lives
  in EncState / SwEncState instead (commit 079611a)
- cap_portal::fourcc() const fn — superseded by drm_fourcc::DrmFourcc
- PwThreadCtx.fps — destructured as `fps: _`, never consumed

Lifetime/ownership invariants (kept with #[allow(dead_code)] + reason):
- CapPortal.rt — ashpd caches a zbus::Connection in a process-global
  OnceCell; runtime must outlive CapPortal or the connection hangs
- EncState.hw_device_ctx — root AVHWDeviceContext; consumers hold their
  own ref_clone() but the root ref must stay alive for ownership

False-positive warning (annotated):
- state_portal use AsRawFd — required at FFI boundary but rustc mis-attributes
  the call to OwnedFd's inherent method; E0599 if removed

Cosmetic:
- drop 5 redundant `as *const i32` casts on linesize.as_ptr() in
  avhw/encode.rs and bin/vaapi_import_bench.rs

src/state.rs

@@ -77,8 +77,6 @@ pub trait CaptureSource: Sized + 'static {
 pub struct OutputInfo {
     pub name: String,
     pub transform: Transform,
-    pub physical_size: (i32, i32),
-    pub logical_position: (i32, i32),
 }
 
 #[derive(Default)]
@@ -174,7 +172,6 @@ pub(crate) enum EncConstructionStage<S: CaptureSource> {
         dmabuf: ZwpLinuxDmabufV1,
     },
     Streaming {
-        output_info: OutputInfo,
         output: WlOutput,
         enc: StreamingEncoder,
         cap: S,
@@ -209,7 +206,6 @@ pub enum InFlightSurface<S: CaptureSource> {
 pub struct State<S: CaptureSource> {
     pub(crate) stage: EncConstructionStage<S>,
     pub in_flight_surface: InFlightSurface<S>,
-    pub starting_timestamp: Option<i64>,
     pub stats_start_time: Option<Instant>,
     pub stats_last_time: Option<Instant>,
     pub stats_frames: u64,
@@ -302,7 +298,6 @@ impl<S: CaptureSource> State<S> {
                 wlr_head_proxy_to_name: HashMap::new(),
             },
             in_flight_surface: InFlightSurface::None,
-            starting_timestamp: None,
             stats_start_time: None,
             stats_last_time: None,
             stats_frames: 0,
@@ -479,7 +474,6 @@ impl<S: CaptureSource> State<S> {
     pub fn on_frame_allocd(&mut self, frame: S::Frame, format: u32, width: u32, height: u32) {
         let (frames_rgb_ctx, dmabuf, cap) = match &mut self.stage {
             EncConstructionStage::Streaming {
-                output_info: _,
                 output: _,
                 enc,
                 dmabuf,
@@ -829,7 +823,6 @@ impl<S: CaptureSource> State<S> {
             bitrate
         );
         self.stage = EncConstructionStage::Streaming {
-            output_info,
             output,
             enc,
             cap,
@@ -985,8 +978,6 @@ impl<S: CaptureSource> State<S> {
                 .or(info.wl_name.clone())
                 .unwrap_or_else(|| format!("output-{}", output_names[target_idx])),
             transform: info.transform.unwrap(),
-            physical_size: info.physical_size.unwrap(),
-            logical_position: info.logical_position.unwrap_or((0, 0)),
         };
         let output = bound_outputs[target_idx].clone();