Add a Kiwix search client and score retrieval (Vikunja #403)

Step one of letting Maven read instead of recall. No LLM yet.

internal/kiwix/client.go: search a local Kiwix server, parse the RSS
reply, hand back title + path + plain-text snippet + word count. The
snippet is the unit of context; a full article is ~100KB of HTML and
will not fit a 4096 token window.

internal/kiwix/retrieval_eval.go plus knowledge_v1.json: the 9 knowledge
questions from the phrasing fixture, each with hand-written English
keywords, scored on whether a wanted article comes back in the top 5.
Opt-in via MAVEN_KIWIX_URL, since CI has no Kiwix. No pass bar, the
number is the finding.

Result on the live mirror: 8/8 answerable questions hit, 7 of them at
rank 1. Retrieval works. Keywords are written by hand on purpose, since
Kiwix ranks by keyword and not by meaning, so a natural question fails.
A query-rewrite step is the next piece of work.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CGeSZxh1DCtRxmFVSYVGvJ

internal/kiwix/knowledge_v1.json

@@ -0,0 +1,63 @@
+{
+  "name": "kiwix-knowledge-v1",
+  "book": "wikipedia_en_all_maxi_2026-02",
+  "note": "The 9 knowledge cases from internal/phraser/eval/talk_v1.json. Queries are hand-written English keywords on purpose: Kiwix ranks by keyword, not meaning, so a natural question fails. Writing them by hand separates 'retrieval is broken' from 'the model writes bad queries'.",
+  "cases": [
+    {
+      "id": "know-sky-blue",
+      "question": "почему небо синее?",
+      "query": "Rayleigh scattering sky blue",
+      "want_titles": ["Rayleigh scattering", "Diffuse sky radiation"]
+    },
+    {
+      "id": "know-boil-egg",
+      "question": "сколько варить яйцо вкрутую?",
+      "query": "boiled egg cooking",
+      "want_titles": ["Boiled egg", "Egg as food"]
+    },
+    {
+      "id": "know-ssd-vs-hdd",
+      "question": "чем ssd отличается от hdd?",
+      "query": "solid-state drive",
+      "want_titles": ["Solid-state drive", "Hard disk drive"]
+    },
+    {
+      "id": "know-cat-purr",
+      "question": "почему кошки мурчат?",
+      "query": "cat purr",
+      "want_titles": ["Purr", "Cat communication"]
+    },
+    {
+      "id": "know-hiccups",
+      "question": "как быстро избавиться от икоты?",
+      "query": "hiccup",
+      "want_titles": ["Hiccup"]
+    },
+    {
+      "id": "know-polite-form",
+      "question": "не могли бы вы объяснить, что такое vpn?",
+      "query": "virtual private network",
+      "want_titles": ["Virtual private network"]
+    },
+    {
+      "id": "know-dont-know",
+      "question": "как зовут моего соседа снизу?",
+      "query": "name of my downstairs neighbour",
+      "want_titles": [],
+      "expect_miss": true,
+      "note": "Unanswerable by design. Retrieval SHOULD find nothing useful. Counted as a hit only when nothing relevant comes back."
+    },
+    {
+      "id": "know-water-per-day",
+      "question": "сколько воды в день надо пить?",
+      "query": "human daily water requirement drinking",
+      "want_titles": ["Drinking water", "Water", "Dehydration", "Hydration"]
+    },
+    {
+      "id": "know-thunder-delay",
+      "question": "почему гром слышно позже молнии?",
+      "query": "thunder speed of sound lightning",
+      "want_titles": ["Thunder", "Lightning"]
+    }
+  ]
+}