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_test.go

@@ -0,0 +1,45 @@
+package memory
+
+import "testing"
+
+func TestConfidentScores(t *testing.T) {
+	cases := []struct {
+		name      string
+		scores    []float64
+		minScore  float64
+		minMargin float64
+		want      bool
+	}{
+		{"no hits", nil, 0.55, 0.008, false},
+		{"below the floor", []float64{0.40, 0.10}, 0.55, 0.008, false},
+		{"clear winner", []float64{0.86, 0.70}, 0.55, 0.008, true},
+		{"runner-up too close", []float64{0.860, 0.858}, 0.55, 0.008, false},
+		// The rule is "beats the runner-up by MORE than delta". Not testing an
+		// exactly-equal margin: no pair of these decimals subtracts to exactly
+		// 0.008 in binary float, so such a test would pin rounding, not the rule.
+		{"margin just under delta", []float64{0.8079, 0.8}, 0.55, 0.008, false},
+		{"margin just over delta", []float64{0.8081, 0.8}, 0.55, 0.008, true},
+		// One hit: nothing to compare against, so only the floor applies.
+		{"single hit clears", []float64{0.86}, 0.55, 0.008, true},
+		{"single hit below floor", []float64{0.10}, 0.55, 0.008, false},
+		// Margin off — the old absolute-only behaviour.
+		{"margin off admits a tie", []float64{0.86, 0.86}, 0.55, 0, true},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			if got := ConfidentScores(c.scores, c.minScore, c.minMargin); got != c.want {
+				t.Errorf("got %v, want %v", got, c.want)
+			}
+		})
+	}
+}
+
+func TestConfidentReadsResultScores(t *testing.T) {
+	res := []Result{{ID: "a", Score: 0.86}, {ID: "b", Score: 0.858}}
+	if Confident(res, 0.55, 0.008) {
+		t.Error("thin margin passed the gate")
+	}
+	if !Confident(res, 0.55, 0) {
+		t.Error("margin off should fall back to the floor alone")
+	}
+}