catalog: fill sensible field defaults for today's engines

Pre-fill the bare input textareas with format-demonstrating samples
(dia2-2b/dia2-1b: [S1]/[S2] dialogue + a nonverbal; csm: conversational;
zonos: expressive multilingual) and give csm.topk a standard default (50).
Addresses asset-engine-dev's best-practice-defaults request so the picker
forms pre-fill usefully instead of empty. No version bumps — these entries
ship fresh in this batch (no prior consumer-pinned shape).

docs/asset-engine/services.yaml

@@ -1268,6 +1268,7 @@ services:
         label: Text ([S1]/[S2] dialogue + nonverbals)
         required: true
         max_length: 5000
+        default: "[S1] Did it actually work? [S2] (laughs) Worked on the first try."
         description: >
           [S1]/[S2] tags mark speaker turns; nonverbals like (laughs),
           (coughs), (sighs), (clears throat) go inline.
@@ -1346,6 +1347,7 @@ services:
         label: Text ([S1]/[S2] dialogue + nonverbals)
         required: true
         max_length: 5000
+        default: "[S1] Did it actually work? [S2] (laughs) Worked on the first try."
         description: >
           [S1]/[S2] tags mark speaker turns; nonverbals like (laughs),
           (coughs), (sighs), (clears throat) go inline.
@@ -1424,6 +1426,7 @@ services:
         label: Text
         required: true
         max_length: 5000
+        default: "Hey, good to finally talk with you. How has your day been?"
       - name: voice
         type: select
         label: Voice
@@ -1450,6 +1453,7 @@ services:
       - name: topk
         type: number
         required: false
+        default: 50
         description: Top-k sampling cutoff (1–100).
       - name: max_audio_length_ms
         type: number
@@ -1526,6 +1530,7 @@ services:
         section: text
         required: true
         max_length: 500
+        default: "This is a quick test of an expressive, multilingual voice."
         description: eSpeak-phonemized. Upstream Gradio caps the box at ~500 chars.
       - name: language
         type: select