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- cap_portal::fourcc() const fn — superseded by drm_fourcc::DrmFourcc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_portal.rs

@@ -2,6 +2,7 @@
 // AsRawFd is required by frame.fd.as_raw_fd() in build_drm_descriptor below
 // but rustc emits a false "unused_imports" warning because OwnedFd also has
 // an inherent as_raw_fd — same quirk as avhw.rs. E0599 if removed → keep it.
+#[allow(unused_imports)]
 use std::os::fd::AsRawFd;
 use std::path::PathBuf;
 use std::sync::atomic::{AtomicBool, Ordering};