[DNM][DNM][WIP] Projectile backend code port and update

Ports#54924,  #65061, #59804 from /tg.
adds auxiliary code from /tg to make code work.

code/game/atoms.dm

@@ -9,6 +9,9 @@
 	plane = GAME_PLANE
 	appearance_flags = TILE_BOUND
 
+	/// pass_flags that we are. If any of this matches a pass_flag on a moving thing, by default, we let them through.
+	var/pass_flags_self = NONE
+
 	var/level = 2
 	///If non-null, overrides a/an/some in all cases
 	var/article
@@ -269,12 +272,27 @@
 		return FALSE
 	if((P.flag in list("bullet", "bomb")) && P.ricochet_incidence_leeway)
 		if((a_incidence_s < 90 && a_incidence_s < 90 - P.ricochet_incidence_leeway) || (a_incidence_s > 270 && a_incidence_s -270 > P.ricochet_incidence_leeway))
-			return
+			return FALSE
 	var/new_angle_s = SIMPLIFY_DEGREES(face_angle + incidence_s)
 	P.setAngle(new_angle_s)
 	return TRUE
 
 /atom/proc/CanPass(atom/movable/mover, turf/target)
+	SHOULD_CALL_PARENT(TRUE)
+	if(mover.movement_type & PHASING)
+		return TRUE
+	. = CanAllowThrough(mover, target)
+	// This is cheaper than calling the proc every time since most things dont override CanPassThrough
+	if(!mover.generic_canpass)
+		return mover.CanPassThrough(src, target, .)
+
+/// Returns true or false to allow the mover to move through src
+/atom/proc/CanAllowThrough(atom/movable/mover, turf/target)
+	SHOULD_CALL_PARENT(TRUE)
+	if(mover.pass_flags & pass_flags_self)
+		return TRUE
+	if(mover.throwing && (pass_flags_self & LETPASSTHROW))
+		return TRUE
 	return !density
 
 /**
@@ -445,9 +463,19 @@
 		wires.emp_pulse(severity)
 	return protection // Pass the protection value collected here upwards
 
-/atom/proc/bullet_act(obj/item/projectile/P, def_zone)
+/**
+ * React to a hit by a projectile object
+ *
+ * Default behaviour is to send the [COMSIG_ATOM_BULLET_ACT] and then call [on_hit][/obj/item/projectile/proc/on_hit] on the projectile
+ *
+ * @params
+ * P - projectile
+ * def_zone - zone hit
+ * piercing_hit - is this hit piercing or normal?
+ */
+/atom/proc/bullet_act(obj/item/projectile/P, def_zone, piercing_hit = FALSE)
 	SEND_SIGNAL(src, COMSIG_ATOM_BULLET_ACT, P, def_zone)
-	. = P.on_hit(src, 0, def_zone)
+	. = P.on_hit(src, 0, def_zone, piercing_hit)
 
 //used on altdisarm() for special interactions between the shoved victim (target) and the src, with user being the one shoving the target on it.
 // IMPORTANT: if you wish to add a new own shove_act() to a certain object, remember to add SHOVABLE_ONTO to its obj_flags bitfied var first.