fix: live-API corrections — music result shapes, mesh format honesty, mesh conversion

Round 2 from live smokes on netherstorm (2026-07-14):

- ACE-Step result blob is an ARRAY of objects and carries RAW control
  characters inside string values (literal newlines) — strict JSON
  rejected it. parseMusicResult sanitizes control chars (only legal
  inside string values in the double-encoded blob) and accepts array or
  object shapes. Regression test uses the live payload shape.
- Hunyuan3D GenerationRequest has NO output-format field (the documented
  type param is fiction) — it always returns GLB. Results are now
  labelled by sniffed magic bytes, never by the requested format.
- NEW meshgen.Converter/ConverterProvider optional surface + llamaswap
  impl over the mediautils shim POST /v1/convert_mesh — the STL hop for
  the printer pipeline.

Co-Authored-By: Claude Fable 5 <[email protected]>

provider/llamaswap/music.go

@@ -72,6 +72,36 @@ type queryItem struct {
 	Result string `json:"result"`
 }
 
+// musicResult is the useful subset of ACE-Step's double-encoded result
+// blob.
+type musicResult struct {
+	File string `json:"file"`
+}
+
+// parseMusicResult decodes the `result` string, tolerating two live-API
+// quirks (observed 2026-07-14): the payload is an ARRAY of result objects
+// (not a bare object), and string values can contain RAW control
+// characters (a literal newline in timing fields) that strict JSON
+// rejects. Control chars can only legally sit inside string values in the
+// double-encoded blob, so replacing them with spaces preserves structure.
+func parseMusicResult(blob string) (musicResult, bool) {
+	sanitized := strings.Map(func(r rune) rune {
+		if r < 0x20 {
+			return ' '
+		}
+		return r
+	}, blob)
+	var arr []musicResult
+	if err := json.Unmarshal([]byte(sanitized), &arr); err == nil && len(arr) > 0 {
+		return arr[0], true
+	}
+	var one musicResult
+	if err := json.Unmarshal([]byte(sanitized), &one); err == nil {
+		return one, true
+	}
+	return musicResult{}, false
+}
+
 // musicFormatMIME resolves the clip MIME from the response Content-Type
 // or the requested format, reusing speechMIME's table (one format->MIME
 // map for the whole provider). wav32 is ACE-Step-specific: normalize it
@@ -232,10 +262,8 @@ func findQueryItem(raw json.RawMessage, taskID string) (*queryItem, error) {
 
 // fetchResult downloads the finished clip named by the job's result blob.
 func (m *musicModel) fetchResult(ctx context.Context, format string, item *queryItem) (*musicgen.Result, error) {
-	var result struct {
-		File string `json:"file"`
-	}
-	if err := json.Unmarshal([]byte(item.Result), &result); err != nil || result.File == "" {
+	result, ok := parseMusicResult(item.Result)
+	if !ok || result.File == "" {
 		return nil, &llm.APIError{Provider: m.p.name, Model: m.id,
 			Message: "music result blob missing file URL: " + truncateForError([]byte(item.Result))}
 	}