feat(host/windows): openh264 software H.264 encoder (GPU-less path)

Windows Encoder impl via the openh264 crate (statically-bundled, BSD-2): low-latency screen-content config (Baseline/no-B-frames, bitrate RC, BT.709 limited, near-infinite GOP + forced-IDR recovery via request_keyframe), packed CPU pixels (BGRx/BGRA/RGB/RGBA/RGBx/BGR) -> I420 -> AnnexB with in-band SPS/PPS each IDR. Synchronous: submit encodes immediately, poll hands back the one AU, flush is a no-op. Windows open_video factory selects it (PUNKTFUNK_ENCODER=software|nvenc|auto; NVENC arm lands later), H.264-only with a clear error otherwise, SW bitrate ceiling. Unit-tested live on the VM: synthetic BGRx -> AnnexB IDR + SPS NAL. Unblocks the GPU-less capture->encode->FEC->send pipeline. Compiles clean on Windows + Linux.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/Cargo.toml

@@ -117,3 +117,6 @@ windows = { version = "0.62", features = [
     "Win32_UI_WindowsAndMessaging",
     "Win32_System_StationsAndDesktops",
 ] }
+# Software H.264 encoder (GPU-less path + NVENC fallback). The default `source` feature statically
+# compiles OpenH264 (BSD-2) — no system lib, builds on MSVC; nasm on PATH adds the SIMD fast path.
+openh264 = "0.9"