feat: add hard mode and weekly mutation system

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

game/mutation.go

@@ -0,0 +1,41 @@
+package game
+
+import (
+	"crypto/sha256"
+	"encoding/binary"
+	"fmt"
+	"time"
+
+	"github.com/tolelom/catacombs/config"
+)
+
+// Mutation represents a weekly gameplay modifier.
+type Mutation struct {
+	ID          string
+	Name        string
+	Description string
+	Apply       func(cfg *config.GameConfig)
+}
+
+// Mutations is the list of all available mutations.
+var Mutations = []Mutation{
+	{ID: "no_skills", Name: "Skill Lockout", Description: "Class skills are disabled",
+		Apply: func(cfg *config.GameConfig) { cfg.SkillUses = 0 }},
+	{ID: "speed_run", Name: "Speed Run", Description: "Turn timeout halved",
+		Apply: func(cfg *config.GameConfig) { cfg.TurnTimeoutSec = max(cfg.TurnTimeoutSec/2, 2) }},
+	{ID: "no_shop", Name: "Shop Closed", Description: "Shops are unavailable",
+		Apply: func(cfg *config.GameConfig) {}},
+	{ID: "glass_cannon", Name: "Glass Cannon", Description: "Double damage, half HP",
+		Apply: func(cfg *config.GameConfig) {}},
+	{ID: "elite_flood", Name: "Elite Flood", Description: "All monsters are elite",
+		Apply: func(cfg *config.GameConfig) {}},
+}
+
+// GetWeeklyMutation returns the mutation for the current week,
+// determined by a SHA-256 hash of the year and ISO week number.
+func GetWeeklyMutation() Mutation {
+	year, week := time.Now().ISOWeek()
+	h := sha256.Sum256([]byte(fmt.Sprintf("mutation:%d:%d", year, week)))
+	idx := int(binary.BigEndian.Uint64(h[:8]) % uint64(len(Mutations)))
+	return Mutations[idx]
+}