ITEM6 B0: golden fixtures + fixture validation skeleton for sing-box migration

26 cases (protocol x representative transport): current-shape input,
expected sing-box fragment, and share-link baseline generated by running
current main@8e89960 GetLink. Blocked/partial cases (xhttp, kcp,
fallbacks, finalmask, vlessenc, removed SS ciphers, tunnel) record their
unmapped concepts under _b0 for the Q1 decision instead of fabricating
output. Link-route baselines only: JSON/Clash subscription families are
being removed wholesale per the 2026-10-23 decision, Q2 closed.

internal/singbox/testdata/golden/tun/expected-singbox.json

@@ -0,0 +1,39 @@
+{
+  "inbounds": [
+    {
+      "type": "tun",
+      "tag": "inbound-0",
+      "interface_name": "xray0",
+      "address": [
+        "172.19.0.1/28"
+      ],
+      "mtu": 1500,
+      "auto_route": true,
+      "route_address": [
+        "0.0.0.0/0"
+      ],
+      "stack": "system"
+    }
+  ],
+  "_b0": {
+    "disposition": "direct",
+    "risk": "Q5",
+    "unmapped": [
+      {
+        "concept": "settings.dns",
+        "input": [
+          "1.1.1.1"
+        ],
+        "reason": "tun 入站本体无 dns 字段，由 sing-box 独立 dns 配置承载——属基础配置面而非入站面，B2 定落点，此处登记防丢。",
+        "pending": "B2"
+      },
+      {
+        "concept": "settings.userLevel",
+        "input": 0,
+        "reason": "xray policy level 概念，sing-box 无对应；本样本 0 为默认，实际无损。",
+        "pending": "Q1"
+      }
+    ],
+    "notes": "gateway[] → address[]；autoSystemRoutingTable → route_address + auto_route；autoOutboundsInterface=auto 省略（sing-box auto_redirect/出口自动选择为近似，字段落位 B1 定）。是否挂 agent 承载属 Q5。"
+  }
+}