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- cap_portal::fourcc() const fn — superseded by drm_fourcc::DrmFourcc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/avhw/encode.rs

@@ -173,7 +173,7 @@ impl SwEncEncode {
                 0,
                 self.enc_height as i32,
                 (*self.yuv_frame).data.as_ptr() as *mut *mut u8,
-                (*self.yuv_frame).linesize.as_ptr() as *const i32,
+                (*self.yuv_frame).linesize.as_ptr(),
             );
             if scaled < 0 {
                 bail!("sws_scale failed for software encoder: {scaled}");

src/avhw/hardware.rs

@@ -17,6 +17,10 @@ pub struct EncState {
     enc_video: ff::codec::encoder::video::Video,
     frames_rgb: AvHwFrameCtx,
     video_filter: ff::filter::Graph,
+    // Root AVHWDeviceContext, kept for ownership. Each consumer (encoder,
+    // filter graph, frames ctx) already holds its own ref_clone(); this
+    // field is never read after `new()` but must outlive those clones.
+    #[allow(dead_code)]
     hw_device_ctx: AvHwDevCtx,
     octx: ff::format::context::Output,
     starting_timestamp: Option<i64>,