refactor(host): hoist shared libav glue into encode/libav.rs (pixel_to_av + swscale consts)

First step of the W2 libav de-dup (plan §2.2, the missing Tier-2 mid-layer). The
three libavcodec backends (Linux NVENC, VAAPI, Windows AMF/QSV) each carried a
byte-identical pixel_to_av plus the SWS_POINT / SWS_CS_ITU709 (/SWS_CS_BT2020)
swscale consts. Hoist them into a new encode/libav.rs and import from super::libav.

The module is gated to compile exactly when a libav backend does (linux, or
windows+amf-qsv). Free fns/consts over borrowed handles — no per-frame dyn/alloc,
off the zero-copy path. Verified: Linux cargo check green (linux/mod.rs + vaapi.rs
compile against it); ffmpeg_win.rs is Windows-cfg — same mechanical swap, covered
by Windows CI on push.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-host/src/encode/linux/mod.rs

@@ -21,14 +21,9 @@ use ffmpeg_next as ffmpeg;
 use std::os::raw::c_int;
 use std::ptr;
 
+use super::libav::{pixel_to_av, SWS_CS_ITU709, SWS_POINT};
 use ffmpeg::ffi; // = ffmpeg_sys_next
 
-/// swscale: nearest-neighbour scaler flag (`SWS_POINT`). We never rescale (src dims == dst dims), so
-/// the resampler choice only governs the colour-conversion path; POINT is the cheapest.
-const SWS_POINT: c_int = 0x10;
-/// swscale colorspace id for ITU-R BT.709 (`SWS_CS_ITU709`) — the CSC coefficients for our RGB→YUV.
-const SWS_CS_ITU709: c_int = 1;
-
 /// The swscale *source* pixel format for a captured packed RGB/BGR layout (the real byte order, not
 /// the NVENC-padded `*0` form). Used by the 4:4:4 RGB→YUV444P conversion path. Mirrors the VAAPI
 /// CPU-input mapping; YUV/10-bit inputs can't feed this path (the 4:4:4 session forces packed RGB).
@@ -118,13 +113,6 @@ impl Drop for CudaHw {
     }
 }
 
-/// `ffmpeg::format::Pixel` → raw `AVPixelFormat`.
-fn pixel_to_av(p: Pixel) -> ffi::AVPixelFormat {
-    // `Pixel` is `#[repr(i32)]`-compatible with `AVPixelFormat` (the bindgen enum) via this
-    // documented conversion in ffmpeg-next.
-    ffi::AVPixelFormat::from(p)
-}
-
 /// Map a captured layout to the NVENC input pixel format, and whether a 3→4 byte expand is
 /// needed (packed RGB/BGR have no padding byte; the NVENC `*0` formats do).
 fn nvenc_input(format: PixelFormat) -> (Pixel, bool) {