feat: add daily challenge seed generation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

game/daily.go

@@ -0,0 +1,11 @@
+package game
+
+import (
+	"crypto/sha256"
+	"encoding/binary"
+)
+
+func DailySeed(date string) int64 {
+	h := sha256.Sum256([]byte("catacombs:" + date))
+	return int64(binary.BigEndian.Uint64(h[:8]))
+}

game/daily_test.go

@@ -0,0 +1,42 @@
+package game
+
+import (
+	"math/rand"
+	"testing"
+
+	"github.com/tolelom/catacombs/dungeon"
+)
+
+func TestDailySeed(t *testing.T) {
+	// Same date produces the same seed
+	seed1 := DailySeed("2026-03-25")
+	seed2 := DailySeed("2026-03-25")
+	if seed1 != seed2 {
+		t.Errorf("same date should produce same seed: got %d and %d", seed1, seed2)
+	}
+
+	// Different date produces a different seed
+	seed3 := DailySeed("2026-03-26")
+	if seed1 == seed3 {
+		t.Errorf("different dates should produce different seeds: both got %d", seed1)
+	}
+}
+
+func TestDailyFloorDeterminism(t *testing.T) {
+	seed := DailySeed("2026-03-25")
+
+	// Generate floor twice with the same seed
+	floor1 := dungeon.GenerateFloor(1, rand.New(rand.NewSource(seed)))
+	floor2 := dungeon.GenerateFloor(1, rand.New(rand.NewSource(seed)))
+
+	if len(floor1.Rooms) != len(floor2.Rooms) {
+		t.Fatalf("room count mismatch: %d vs %d", len(floor1.Rooms), len(floor2.Rooms))
+	}
+
+	for i, r1 := range floor1.Rooms {
+		r2 := floor2.Rooms[i]
+		if r1.Type != r2.Type {
+			t.Errorf("room %d type mismatch: %d vs %d", i, r1.Type, r2.Type)
+		}
+	}
+}