feat: 拆分 providers.json 为按 Provider 独立文件

将 compute/providers.json（54KB, 14 providers, 99 models）拆分为
compute/providers/ 目录下的独立 JSON 文件，便于维护和按需更新。

新增文件：
- compute/providers/_index.json — 加载顺序索引（保证 digest 确定性）
- compute/providers/{provider}.json × 14 — 各 provider 独立配置

保留 compute/providers.json 供旧客户端向后兼容（过渡期）。

presetDataVersion: 2 → 3

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

compute/providers/_index.json

@@ -0,0 +1,19 @@
+{
+  "description": "Provider 加载顺序索引",
+  "order": [
+    "openai",
+    "anthropic",
+    "deepseek",
+    "dashscope",
+    "volcengine",
+    "moonshot",
+    "zhipu",
+    "baichuan",
+    "lingyiwanwu",
+    "local-whisper",
+    "ollama",
+    "google",
+    "kling",
+    "stability"
+  ]
+}