feat(homing): CastStats.homing_add + 修饰器折叠 + 发射写归航冷数据 + 分支快照; modifier_homing 词条

homing_add 为 float（角速度，弧度/秒），镜像 bounce_add 的 int 累加路径。
发射时不解析目标，homing_target_id 留 -1 由 bullet_manager 首帧惰性获取，
使 spell_evaluator 不必接触 SpatialGrid。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

data/spells.json

@@ -15,6 +15,7 @@
   "modifier_double_cast": { "type": 1, "display_name": "Double Cast",  "description": "本次施法额外执行 1 个 ACTION。",     "element_tags": [], "meta": { "multicast": 1, "mana_cost": 2, "shop_cost": 25 } },
   "modifier_pierce_plus": { "type": 1, "display_name": "Pierce +2",    "description": "后续法术穿透 +2。",                "element_tags": [], "meta": { "pierce": 2, "mana_cost": 8, "shop_cost": 18 } },
   "modifier_bounce":      { "type": 1, "display_name": "Bounce +2",    "description": "后续法术弹跳 +2（命中后弹向最近未命中敌人，伤害递减）。", "element_tags": [], "meta": { "bounce": 2, "mana_cost": 8, "shop_cost": 18 } },
+  "modifier_homing":      { "type": 1, "display_name": "Homing",      "description": "后续法术获得归航（飞行中以受限角速度转向锁定的敌人）。", "element_tags": [], "meta": { "homing": 3.0, "mana_cost": 8, "shop_cost": 18 } },
   "modifier_heavy_cost":  { "type": 1, "display_name": "Blood Sacrifice", "description": "鲜血献祭：伤害+50，但每次施法扣 10 HP。", "element_tags": [], "meta": { "damage_add": 50.0, "heavy_cost": 10, "mana_cost": 0, "shop_cost": 26 } },
   "modifier_mana_leech":  { "type": 1, "display_name": "Mana Leech",      "description": "魔力虹吸：击杀回复 2 魔力（放入卡组即生效）。", "element_tags": [], "meta": { "mana_leech": 2, "mana_cost": 0, "shop_cost": 22 } },
   "trigger_on_hit":       { "type": 2, "display_name": "Trigger: On Hit","description": "命中时在命中点执行后续法术（子母弹）。", "element_tags": [], "meta": { "base_damage": 3.0, "speed": 250.0, "lifetime": 4.0, "radius": 6.0, "damage_type": 0, "mana_cost": 10, "shop_cost": 30 } },

scripts/domain/spell_system/cast_stats.gd

@@ -13,6 +13,7 @@ var radius_mult:      float = 1.0   # 弹体半径倍率
 var crit_chance:      float = 0.0   # 暴击概率
 var pierce_add:       int   = 0     # MODIFIER 累加的穿透次数（叠加到 ACTION 自带 pierce）
 var bounce_add:       int   = 0     # MODIFIER 累加的弹跳次数（叠加到 ACTION 自带 bounce）
+var homing_add:       float = 0.0   # MODIFIER 累加的归航角速度(弧度/秒，叠加到 ACTION 自带 homing)
 
 func reset() -> void:
 	damage_add      = 0.0
@@ -25,3 +26,4 @@ func reset() -> void:
 	crit_chance     = 0.0
 	pierce_add      = 0
 	bounce_add      = 0
+	homing_add      = 0.0

scripts/domain/spell_system/spell_evaluator.gd

@@ -436,6 +436,11 @@ func _push_projectile(node: SpellNode, ctx: SpellContext, spawn_pos: Vector2, tr
 			cold["visited_targets"]  = []
 			cold["bounce_decay"]     = float(meta.get("bounce_decay", 0.9))
 			cold["bounce_range"]     = float(meta.get("bounce_range", 250.0))
+		var homing: float = float(meta.get("homing", 0.0)) + ctx.stats.homing_add
+		if homing > 0.0:
+			cold["homing_strength"]  = homing
+			cold["homing_range"]     = float(meta.get("homing_range", 400.0))
+			cold["homing_target_id"] = -1
 		# S3: 将 trigger_depth 写入冷数据供子弹命中时使用
 		if trigger_depth >= 0:
 			cold["trigger_depth"] = trigger_depth
@@ -531,6 +536,8 @@ func _apply_modifier(node: SpellNode, ctx: SpellContext) -> void:
 		ctx.stats.pierce_add    += int(meta["pierce"])
 	if meta.has("bounce"):
 		ctx.stats.bounce_add += int(meta["bounce"])
+	if meta.has("homing"):
+		ctx.stats.homing_add += float(meta["homing"])
 	if meta.has("heavy_cost"):
 		ctx.hp_cost_accum += float(meta["heavy_cost"])
 
@@ -612,7 +619,7 @@ func _run_branch_payload(payload_id: int, ctx: SpellContext, spawn_pos: Vector2,
 	var nodes: Array = SubPayloadRegistry.get_nodes(payload_id)
 	if nodes.is_empty():
 		return
-	# 快照分支前 CastStats（9 字段，无分配），分支结束后还原 → 分支局部 MODIFIER 不外泄
+	# 快照分支前 CastStats（10 字段，无分配），分支结束后还原 → 分支局部 MODIFIER 不外泄
 	var s: CastStats = ctx.stats
 	var b_dadd: float = s.damage_add
 	var b_dmul: float = s.damage_mult
@@ -623,6 +630,7 @@ func _run_branch_payload(payload_id: int, ctx: SpellContext, spawn_pos: Vector2,
 	var b_rad:  float = s.radius_mult
 	var b_crit: float = s.crit_chance
 	var b_bnc:  int   = s.bounce_add
+	var b_hom:  float = s.homing_add
 	for node in nodes:
 		var node_cost: float = float(node.meta.get("mana_cost", 0.0))
 		if not _charge_mana(node_cost, ctx, infinite, heat_mult):
@@ -650,6 +658,7 @@ func _run_branch_payload(payload_id: int, ctx: SpellContext, spawn_pos: Vector2,
 	s.radius_mult     = b_rad
 	s.crit_chance     = b_crit
 	s.bounce_add      = b_bnc
+	s.homing_add      = b_hom
 
 func _get_aim_direction(from_pos: Vector2) -> Vector2:
 	var target: Vector2 = EnemyManager.get_nearest_pos(from_pos, 9999.0)