fix(goLive): encode H264 baseline to match SDP profile-level-id (black tile)

SDP offer advertises profile-level-id=42e01f (constrained baseline) but
x264 encoded the default High profile — Discord's receiver configures its
decoder from the negotiated profile, so the High-profile bitstream failed
to decode → black GoLive tile despite valid access units + correct RTP
timestamps (fixed in 42a503c).

- Add -profile:v baseline to H264 encoder options (matches @dank074's
  proven config; SPS now 6742c01e → profile_idc=66 baseline, aligns with
  the 42e01f fmtp).
- Default x264 tune film → zerolatency (no lookahead — correct for live
  GoLive; @dank074 uses it).
- Update goLive-port test to assert baseline + zerolatency.

services/discord-gateway/src/goLive/Encoders.ts

@@ -26,13 +26,24 @@ export function software(
   } = {},
 ): () => EncoderSet {
   const { x264, x265 } = opts;
-  const { preset: x264Preset = "superfast", tune: x264Tune = "film" } =
+  const { preset: x264Preset = "superfast", tune: x264Tune = "zerolatency" } =
     x264 ?? {};
   const { preset: x265Preset = "superfast", tune: x265Tune } = x265 ?? {};
   return () => ({
     H264: {
       name: "libx264",
-      options: ["-forced-idr 1", `-tune ${x264Tune}`, `-preset ${x264Preset}`],
+      // -profile:v baseline is REQUIRED: the SDP advertises
+      // profile-level-id=42e01f (constrained baseline) and Discord's
+      // receiver decodes with that profile. x264's default is High — a
+      // High-profile bitstream against a baseline SDP negotiation fails to
+      // decode → black GoLive tile (production bug, fixed 2026-08-12).
+      // zerolatency matches @dank074 (no lookahead — correct for live).
+      options: [
+        "-forced-idr 1",
+        "-profile:v baseline",
+        `-tune ${x264Tune}`,
+        `-preset ${x264Preset}`,
+      ],
     },
     H265: {
       name: "libx265",