Gate recall on the margin over the runner-up, not just the score

The e5 embedder puts every cosine in one narrow band (0.79-0.89), so the
absolute query_min_score gate cannot tell a real hit from a made-up
question: any value under the band answers everything, any value above it
answers nothing. False recall was 5/5.

New gate asks whether one note is clearly the best instead: top1 - top2 >
delta. New query_min_margin config knob, default 0.008, read off the sweep
in the recall harness. The absolute floor stays as a second check.

On the recall fixture with e5: answered 72% -> 68%, false recall 5/5 -> 1/5.

Vikunja #359

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

internal/memory/gate.go

@@ -0,0 +1,38 @@
+package memory
+
+// Confidence gate for a recall. Two checks, both must pass before Maven says a
+// note back:
+//
+//   - minScore — an absolute cosine floor.
+//   - minMargin — the top hit must beat the runner-up by more than this.
+//
+// The margin is the one that carries the weight. The e5 embedder packs every
+// score into a narrow high band (0.79-0.89 on the recall fixture), so an
+// absolute floor cannot tell a real hit from a confident-looking miss: every
+// value under the band admits everything, every value above it answers nothing.
+// A margin asks a different question — "is this note clearly the best one, or
+// is the whole shelf equally close?" — and a made-up question has no clear best.
+//
+// With one hit and no runner-up there is nothing to compare, so only the floor
+// applies.
+
+// ConfidentScores reports whether the top score clears both gates. scores must
+// be sorted highest first. minMargin <= 0 turns the margin check off.
+func ConfidentScores(scores []float64, minScore, minMargin float64) bool {
+	if len(scores) == 0 || scores[0] < minScore {
+		return false
+	}
+	if minMargin > 0 && len(scores) > 1 && scores[0]-scores[1] <= minMargin {
+		return false
+	}
+	return true
+}
+
+// Confident is ConfidentScores for search results.
+func Confident(results []Result, minScore, minMargin float64) bool {
+	scores := make([]float64, len(results))
+	for i, r := range results {
+		scores[i] = r.Score
+	}
+	return ConfidentScores(scores, minScore, minMargin)
+}