feat(host): VAAPI codec probe + AMD/Intel packaging + neutral logs (Phase 3)

Polish for AMD/Intel support:
- GameStream serverinfo advertises only codecs the GPU can ACTUALLY encode on
  the VAAPI backend (probed once by opening a tiny encoder per codec). AV1
  encode is narrow (Intel Arc/Xe2+, AMD RDNA3+/RDNA4) and an old iGPU may lack
  HEVC, so a Moonlight client never negotiates a codec the encoder can't open.
  NVENC/Windows keep the Moonlight-validated static mask. Validated on a Radeon
  780M: h264/h265/av1 all probe true -> mask unchanged (65793).
- Packaging: Recommends mesa-va-drivers + intel-media-va-driver (deb) /
  mesa-va-drivers + intel-media-driver (rpm) so the auto-selected VAAPI backend
  works out of the box on AMD/Intel; NVIDIA boxes can --no-install-recommends.
  (Fedora note: stock mesa-va-drivers disables HEVC/AV1 -- needs the freeworld
  variant from RPM Fusion.)
- De-NVIDIA-fy the user-facing encoder log/context strings ("open NVENC" ->
  "open video encoder") now that VAAPI is a first-class backend.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/punktfunk-host/src/gamestream/stream.rs

@@ -295,7 +295,7 @@ fn stream_body(
         frame.is_cuda(),
         8, // GameStream/Moonlight path: 8-bit (its own codec negotiation)
     )
-    .context("open NVENC for stream")?;
+    .context("open video encoder for stream")?;
     // FEC overhead percent (Sunshine default 20). Override with PUNKTFUNK_FEC_PCT (0 = data-only).
     let fec_pct: u8 = std::env::var("PUNKTFUNK_FEC_PCT")
         .ok()