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/mesh_convert.go

@@ -0,0 +1,64 @@
+// mesh_convert.go implements meshgen.ConverterProvider against the
+// mediautils shim's POST /v1/convert_mesh (multipart file + target),
+// reached through the /upstream passthrough (ADR-0020). Exists because
+// Hunyuan3D's api_server always returns GLB — STL for the printer
+// pipeline is produced by this conversion hop.
+package llamaswap
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"strings"
+
+	"gitea.stevedudenhoeffer.com/steve/majordomo/llm"
+	"gitea.stevedudenhoeffer.com/steve/majordomo/meshgen"
+)
+
+// MeshConverter implements meshgen.ConverterProvider.
+func (p *Provider) MeshConverter(id string) (meshgen.Converter, error) {
+	if err := p.requireBaseURL(); err != nil {
+		return nil, err
+	}
+	return &meshConverter{p: p, id: id}, nil
+}
+
+type meshConverter struct {
+	p  *Provider
+	id string
+}
+
+// Convert implements meshgen.Converter.
+func (m *meshConverter) Convert(ctx context.Context, mesh meshgen.Mesh, format string) (*meshgen.Result, error) {
+	if len(mesh.Data) == 0 {
+		return nil, fmt.Errorf("%w: mesh conversion requires mesh bytes", llm.ErrUnsupported)
+	}
+	format = strings.ToLower(strings.TrimSpace(format))
+	mimeType, ok := meshFormats[format]
+	if !ok {
+		return nil, fmt.Errorf("%w: unsupported mesh format %q (want glb, stl, or obj)", llm.ErrUnsupported, format)
+	}
+	path, err := upstreamPath(m.id, "/v1/convert_mesh")
+	if err != nil {
+		return nil, err
+	}
+	filename := "mesh." + mesh.Format
+	if mesh.Format == "" {
+		filename = "mesh"
+	}
+	body, contentType, err := buildMultipart("build mesh-convert form",
+		filePart{field: "file", filename: filename, data: mesh.Data},
+		[]formField{{"target", format, true}})
+	if err != nil {
+		return nil, err
+	}
+	raw, _, err := m.p.doRaw(ctx, http.MethodPost, path, m.id, contentType, body, maxMeshResponseBytes)
+	if err != nil {
+		return nil, err
+	}
+	if len(raw) == 0 {
+		return nil, &llm.APIError{Provider: m.p.name, Model: m.id, Message: "mesh conversion returned no data"}
+	}
+	actualFormat, actualMIME := sniffMeshFormat(raw, format, mimeType)
+	return &meshgen.Result{Mesh: meshgen.Mesh{Data: raw, Format: actualFormat, MIME: actualMIME}}, nil
+}