refactor(bin): dedupe av_err_to_string + receive_first_frame + drain_encoder via shared src/bin/common/mod.rs

Step 2a: eliminate cross-bench duplication identified by the Explore audit.

Changes:
- src/avhw/util.rs: av_err_to_string promoted pub(crate) -> pub (the only
  change to src/avhw/ in this whole refactor plan).
- src/avhw/mod.rs: re-export av_err_to_string; #[allow(unused_imports)]
  silences rustc's per-bin unused-import false positive (the pub use is
  consumed by the bench bins, not by the main bin).
- src/bin/common/mod.rs (new): shared receive_first_frame + drain_encoder.
  These were byte-identical between the two bench binaries modulo a type-path
  alias (ff::codec::encoder::video::Video vs ff::encoder::video::Video) and
  SAFETY-comment line wrapping. Both binaries now wire it via
  #[path = "common/mod.rs"] mod common;.
- src/bin/vaapi_import_bench.rs: 1039 -> 947 LOC (av_err_to_string,
  receive_first_frame, drain_encoder all removed; 3 call sites updated).
- src/bin/sw_encode_bench.rs: 614 -> 545 LOC (receive_first_frame,
  drain_encoder removed; 3 call sites updated).
- use ffmpeg_next::packet::Mut moved to common/mod.rs (was needed only for
  pkt.as_mut_ptr() inside drain_encoder).

Verification (all green):
- cargo build --bins / cargo build --release
- cargo test (79 lib + 3 integration = 82 pass, 1 ignored — unchanged)
- cargo clippy --all-targets -- -D warnings
- cargo fmt --check
- Test counts unchanged from baseline

src/avhw/mod.rs

@@ -63,6 +63,8 @@ pub use types::{
     BitrateCommand, CpuNv12Frame, EncodeOutcome, EncodeStages, EncodedH264Frame, ResolutionChange,
     SwEncodeTiming,
 };
+#[allow(unused_imports)]
+pub use util::av_err_to_string;
 pub(crate) use util::ff_err;
 
 // ---------------------------------------------------------------------------

src/avhw/util.rs

@@ -1,7 +1,7 @@
 use ffmpeg_next::ffi;
 
 /// Convert an FFmpeg error code to a human-readable string.
-pub(crate) fn av_err_to_string(err: i32) -> String {
+pub fn av_err_to_string(err: i32) -> String {
     let mut buf = vec![0u8; 128];
     // SAFETY: buf points to 128 writable bytes and lives for the duration of
     // av_strerror.