catalog(dia2): default to a working stable voice (fix undefined-ref 404)

Reported: dia2-1b 404 'Reference audio file not found: undefined' when
accepting defaults — voice_mode=clone with no clone_reference_filename made
the UI submit the literal 'undefined'. Fix: voice_mode now defaults to clone
AND clone_reference_filename defaults to Abigail.wav (a staged voice), so the
out-of-box request is a valid, stable voice. Reproduced the 404 and verified
the Abigail.wav path returns 200. Folded into v2 (not yet consumed downstream).

docs/asset-engine/services.yaml

@@ -1285,17 +1285,20 @@ services:
         type: select
         label: Voice mode
         section: basic
-        options: [single_s1, single_s2, dialogue, clone]
-        default: single_s1
+        options: [clone, single_s1, single_s2, dialogue]
+        default: clone
         description: >
-          Canonical default single_s1. Unconditioned modes (single_s1/s2,
-          dialogue) sample a RANDOM speaker each call — gender varies. For a
-          STABLE voice, set this to clone and pick a clone_reference_filename.
+          Defaults to `clone` -> a stable predefined voice (clone_reference_filename
+          below, default Abigail.wav). The unconditioned modes (single_s1/s2,
+          dialogue) sample a RANDOM speaker each call (gender varies) — pick one
+          only if you want that. NOTE: voice_mode=clone REQUIRES a real
+          clone_reference_filename; an empty/"undefined" value 404s.
       - name: clone_reference_filename
         type: select
         label: Voice (clone reference)
         section: basic
         optional: true
+        default: "Abigail.wav"
         source_url: http://10.100.79.3:8200/get_reference_files
         source_jsonpath: $[*]
         description: >
@@ -1446,17 +1449,20 @@ services:
         type: select
         label: Voice mode
         section: basic
-        options: [single_s1, single_s2, dialogue, clone]
-        default: single_s1
+        options: [clone, single_s1, single_s2, dialogue]
+        default: clone
         description: >
-          Canonical default single_s1. Unconditioned modes (single_s1/s2,
-          dialogue) sample a RANDOM speaker each call — gender varies. For a
-          STABLE voice, set this to clone and pick a clone_reference_filename.
+          Defaults to `clone` -> a stable predefined voice (clone_reference_filename
+          below, default Abigail.wav). The unconditioned modes (single_s1/s2,
+          dialogue) sample a RANDOM speaker each call (gender varies) — pick one
+          only if you want that. NOTE: voice_mode=clone REQUIRES a real
+          clone_reference_filename; an empty/"undefined" value 404s.
       - name: clone_reference_filename
         type: select
         label: Voice (clone reference)
         section: basic
         optional: true
+        default: "Abigail.wav"
         source_url: http://10.100.79.3:8202/get_reference_files
         source_jsonpath: $[*]
         description: >