Bullet field: pierce-through and damaging, rings spaced 3 cells apart

Solid bullets were blocking the patrolling enemies, so the field now defaults to solid = false: each bullet is a plain StaticBody2D with no collision layer, plus an Area2D that only detects the player, so nothing is physically obstructed while contact still costs a heart. Ring radii changed from [5, 10, 15] to [3, 6, 9]. Damage is now applied every frame to whatever is overlapping, not only on body_entered, so standing inside a bullet keeps hurting instead of only the first touch. Also picks up the editor change that removed Walker_03.

scripts/bullet_field.gd.uid

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