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]>

meshgen/meshgen.go

@@ -150,3 +150,23 @@ type Provider interface {
 	// backend verbatim; no catalog validation).
 	MeshModel(id string, opts ...ModelOption) (Model, error)
 }
+
+// Converter re-encodes a mesh between containers (GLB/STL/OBJ). A separate
+// optional surface because conversion typically runs on a DIFFERENT backend
+// than generation (the reference host converts on its mediautils shim —
+// Hunyuan3D's live server always emits GLB regardless of the requested
+// format).
+type Converter interface {
+	// Convert returns the mesh re-encoded in the given format.
+	Convert(ctx context.Context, mesh Mesh, format string) (*Result, error)
+}
+
+// ConverterProvider mints Converters bound to one backend.
+type ConverterProvider interface {
+	// Name is the registry identifier for the provider.
+	Name() string
+
+	// MeshConverter returns a Converter bound to the given id (passed
+	// through to the backend verbatim; no catalog validation).
+	MeshConverter(id string) (Converter, error)
+}