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
