docs(arch): 补删 ProjectileDef.reset() 里遗留的 homing_force 赋值

上一提交删了声明与注释、漏了 31 行后 reset() 内的赋值，
导致同一代码块自相矛盾（注释声明该字段不存在，正文仍在赋值）。

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

docs/technical/architecture_design.md

@@ -230,7 +230,7 @@ var proc_rate: float = 1.0           # Proc 系数（0.0~1.0）。高频攻击
 func reset() -> void:
     base_damage = 5.0; damage_mult = 1.0; damage_type = DamageType.PHYSICAL
     speed = 600.0; direction = Vector2.RIGHT; spread_angle_rad = 0.0
-    homing_force = 0.0; acceleration = 0.0; lifetime = 3.0
+    acceleration = 0.0; lifetime = 3.0
     pierce_remaining = 0; bounce_remaining = 0
     radius = 6.0; size_mult = 1.0
     on_hit_payload_id = -1; on_expire_payload_id = -1; on_kill_payload_id = -1