feat(routing): decision policy

Pure-function Policy{Floor,Ceil} with Decide(*float64, uint64) Decision.
Rules in priority order: nil → local; ≥floor → local; <ceil → claude;
sample band → low bit of requestHash.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/routing/policy_test.go

@@ -0,0 +1,36 @@
+package routing_test
+
+import (
+	"testing"
+
+	"github.com/mathiasbq/supervisor/internal/routing"
+	"github.com/stretchr/testify/assert"
+)
+
+func ptr(f float64) *float64 { return &f }
+
+func TestPolicyDecide(t *testing.T) {
+	p := routing.Policy{Floor: 0.9, Ceil: 0.7}
+
+	cases := []struct {
+		name     string
+		passRate *float64
+		hash     uint64
+		want     routing.Decision
+	}{
+		{"null pass rate → local", nil, 0, routing.DecideLocal},
+		{"null pass rate, hash irrelevant → local", nil, 0xDEADBEEF, routing.DecideLocal},
+		{"at floor → local", ptr(0.9), 0, routing.DecideLocal},
+		{"above floor → local", ptr(0.95), 0, routing.DecideLocal},
+		{"below ceil → claude", ptr(0.5), 0, routing.DecideClaude},
+		{"at ceil → sample-band even-hash → local", ptr(0.7), 0, routing.DecideLocal},
+		{"sample band, even hash → local", ptr(0.8), 2, routing.DecideLocal},
+		{"sample band, odd hash → claude", ptr(0.8), 3, routing.DecideClaude},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			assert.Equal(t, tc.want, p.Decide(tc.passRate, tc.hash))
+		})
+	}
+}