p3: map Brave LLM Context; RRF merge; per-source no_rerank

Live llm/context hits live in grounding.generic plus a URL-keyed
sources map, not a sources array. Fan-in uses reciprocal-rank fusion
instead of cosine. no_rerank keeps that source's own order as a prefix.

tests/testdata/brave_llm_context.json

@@ -0,0 +1,26 @@
+{
+  "grounding": {
+    "generic": [
+      {
+        "url": "https://docs.tavily.com/documentation/api-reference/endpoint/search",
+        "title": "Tavily Search API",
+        "snippets": ["POST /search with query and max_results.", "search_depth basic is one credit."]
+      },
+      {
+        "url": "https://api.search.brave.com/help",
+        "title": "Brave Search API",
+        "snippets": ["LLM Context returns grounding.generic."]
+      }
+    ]
+  },
+  "sources": {
+    "https://docs.tavily.com/documentation/api-reference/endpoint/search": {
+      "title": "Tavily Search API",
+      "hostname": "docs.tavily.com"
+    },
+    "https://orphan.example/only-in-sources": {
+      "title": "Orphan",
+      "hostname": "orphan.example"
+    }
+  }
+}