ace-step: stream audio bytes inline; catalog v3 → v4

The pre-fix wrapper at stacks/ace-step/infer-api.py returned a JSON
{output_path: "..."} reference to a file written inside the
container at /app/outputs/. That path was unreachable from outside
the container — every consumer got 134 bytes of JSON-pretending-to-
be-WAV instead of audio. Surfaced by the asset_engine consumer's
end-to-end smoke (althing thread 01KRCJF7NGMXYE9F62Q1A6KFD4 msg 5);
my own earlier smoke missed it because I checked HTTP=200 and stopped
reading instead of inspecting the response body.

Wrapper now reads back the file the pipeline writes and streams the
bytes via fastapi.responses.Response with media_type set from the
audio_format request field (audio/wav | audio/mpeg | audio/flac).
The in-container path is exposed via X-Output-Path header for log
correlation but is no longer load-bearing.

Verified end-to-end against live ace-step on irv-ml1:
  POST /generate  ->  HTTP 200 in 80s
  content-type: audio/wav
  content-length: 945226
  x-output-path: /app/outputs/output_cfe87d1d....wav
  $ file response.wav
  RIFF (little-endian) data, WAVE audio, Microsoft PCM, 16 bit,
    stereo 48000 Hz

Catalog: ace-step bumped version 3 -> 4. Dropped
response.output_field (no longer applicable). reproducibility.notes
expanded to record both the v2 18-arg-tuple fix and this v4
inline-streaming change so the history is auditable from the
catalog itself.

Stale ACEStepOutput Pydantic model left in infer-api.py for now —
unused but small; future cleanup.

docs/asset-engine/services.yaml

@@ -752,7 +752,7 @@ services:
       Apache-2.0 hybrid diffusion+LLM music generation. Multi-minute lyric-aware
       songs with vocals + instrumentation.
     category: music
-    version: 3
+    version: 4
     host: irv-ml1
     endpoint: http://10.100.79.3:8210/generate
     method: POST
@@ -948,16 +948,23 @@ services:
           Wrapper-side cleanup queued — once the upstream model defaults this,
           the catalog field will become optional or be dropped entirely.
     response:
+      # As of wrapper version that ships with image local/ace-step:v1
+      # post 2026-05-11, /generate streams audio bytes inline with
+      # Content-Type set from the audio_format request field. The
+      # in-container output_path is exposed via X-Output-Path header
+      # for log correlation but is no longer load-bearing.
       type: audio
       mime_from_field: audio_format
-      output_field: output_path
     reproducibility:
       seedable: true
       deterministic: true
       notes: >
         actual_seeds parameter exposed; identical seeds + params = identical audio.
         Local infer-api.py patches upstream's broken 24-arg pipeline signature
-        (was 18 in upstream — caused crashes with audio_duration in `format` slot).
+        (was 18 in upstream — caused crashes with audio_duration in `format` slot)
+        AND inline-streams the generated audio bytes (was returning a JSON
+        path reference to a file inside the container, which was unreachable
+        from outside).
     estimated_latency:
       cold_start_s: 30
       warm_per_unit: "~10–60s depending on audio_duration + infer_step"