fix(kbsearch): rank FTS correctly, filter before -n, start the daemon. (#5)

Go search took worst BM25 hits (ORDER BY score), cut to -n before --root,
and never called ensureDaemon. Ranking and flag parsing move to a cgo-free
package so CI can fail those regressions without ladybug. --hop errors
instead of being swallowed into the query.

bin/kbsearch/rank/rank.go

@@ -0,0 +1,100 @@
+// Package rank is the cgo-free ranking and CLI parsing for kbsearch.
+// CI can `go test ./rank` without the native ladybug library.
+package rank
+
+import (
+	"sort"
+	"strings"
+)
+
+// Hit is one search result, mirroring the python script's dict shape.
+type Hit struct {
+	ID      string  `json:"id"`
+	Text    string  `json:"text"`
+	Root    string  `json:"root"`
+	Source  string  `json:"-"`
+	Score   float64 `json:"score"`
+	Snippet string  `json:"snippet,omitempty"`
+}
+
+// rrfK dampens the contribution of low ranks; same constant as kblib.py.
+const rrfK = 60
+
+// RankAndFilter fuses the two hit lists, applies --root/--repo, then cuts to
+// limit. Cutting first dropped every matching leaf ranked below the cut, so
+// `--root facts` came back empty whenever info leafs filled the top N.
+// limit <= 0 keeps everything.
+func RankAndFilter(fts, vec []Hit, root, repo string, limit int) []Hit {
+	out := Hybrid(fts, vec, 0)
+	if root != "" {
+		out = FilterRoot(out, root)
+	}
+	if repo != "" {
+		out = FilterRepo(out, repo)
+	}
+	if limit > 0 && len(out) > limit {
+		out = out[:limit]
+	}
+	return out
+}
+
+// Hybrid merges FTS and vector hits by reciprocal rank fusion.
+// limit <= 0 returns the full fused list.
+func Hybrid(fts, vec []Hit, limit int) []Hit {
+	byID := make(map[string]Hit, len(fts)+len(vec))
+	rrf := make(map[string]float64, len(fts)+len(vec))
+
+	for i, h := range fts {
+		byID[h.ID] = h
+		rrf[h.ID] += 1.0 / (rrfK + float64(i+1))
+	}
+	for i, h := range vec {
+		if existing, ok := byID[h.ID]; !ok {
+			byID[h.ID] = h
+		} else if existing.Score == 0 {
+			existing.Score = h.Score
+			byID[h.ID] = existing
+		}
+		rrf[h.ID] += 1.0 / (rrfK + float64(i+1))
+	}
+
+	ids := make([]string, 0, len(rrf))
+	for id := range rrf {
+		ids = append(ids, id)
+	}
+	sort.Slice(ids, func(i, j int) bool {
+		if rrf[ids[i]] != rrf[ids[j]] {
+			return rrf[ids[i]] > rrf[ids[j]]
+		}
+		return ids[i] < ids[j]
+	})
+	if limit > 0 && len(ids) > limit {
+		ids = ids[:limit]
+	}
+
+	out := make([]Hit, 0, len(ids))
+	for _, id := range ids {
+		out = append(out, byID[id])
+	}
+	return out
+}
+
+func FilterRoot(hits []Hit, root string) []Hit {
+	var out []Hit
+	for _, h := range hits {
+		if h.Root == root {
+			out = append(out, h)
+		}
+	}
+	return out
+}
+
+func FilterRepo(hits []Hit, repo string) []Hit {
+	var out []Hit
+	for _, h := range hits {
+		if strings.Contains(h.Source, repo) {
+			out = append(out, h)
+		}
+	}
+	return out
+}