Mech Controls Rework (#21762)

This PR reworks the controls for Mechs to now operate under
"Battletech-Style" controls. Where the W and S keys are instead Forward
and Reverse, while A and D are turn-left and turn-right respectively.
This control scheme is generally significantly more playable for mechs,
since mechs can only interact with objects or fire weapons in a
forward-facing arc. Mechs were previously incapable of both
repositioning and fighting at the same time, while with this update a
mech can now face an enemy, fire upon them, and either move closer to
them or further away.

Mechs also now have reverse speed characteristics based on their chassis
selection. Bipedal legs generally have the worst reverse speed. Quad
legs have significantly better reverse handling, and Treads have no
reverse speed reduction at all (with the fun caveat that they suck at
turning).

I have also fixed the issue of mechs feeling like they were "too easy to
mobility kill". It turned out that mechs were hardcoded to be mobility
killed when they took only 45 points of damage, regardless of how tough
their legs were. I have reworked this to instead be a linear decrease in
movement speed, based on the damage taken ratio of the legs. Larger and
tougher legs are naturally more resistant to being impeded by mobility
damage.

I have actually tested this PR, here's it in action! NOW WITH STRAFING


https://github.com/user-attachments/assets/3123fed8-ec22-4118-8a6a-7cbfe45e6667

---------

Signed-off-by: VMSolidus <evilexecutive@gmail.com>
Co-authored-by: Cody Brittain <1779662+Generalcamo@users.noreply.github.com>

code/modules/mob/living/carbon/human/inventory.dm

@@ -7,6 +7,11 @@ This saves us from having to call add_fingerprint() any time something is put in
 	set name = "quick-equip"
 	set hidden = 1
 
+	var/cancelled = FALSE
+	SEND_SIGNAL(src, COMSIG_INPUT_KEY_QUICK_EQUIP, &cancelled)
+	if (cancelled)
+		return
+
 	if(ishuman(src))
 		var/mob/living/carbon/human/H = src
 		var/obj/item/I = H.get_active_hand()

code/modules/mob/mob_movement.dm

@@ -52,6 +52,9 @@
 	diagonal_action(SOUTHWEST)
 
 /client/proc/diagonal_action(direction)
+	if (!mob)
+		return
+
 	switch(client_dir(direction, 1))
 		if(NORTHEAST)
 			swap_hand()
@@ -67,6 +70,11 @@
 				to_chat(usr, SPAN_WARNING("This mob type cannot throw items."))
 			return
 		if(NORTHWEST)
+			var/cancelled = FALSE
+			SEND_SIGNAL(mob, COMSIG_INPUT_KEY_DROP, &cancelled)
+			if (cancelled)
+				return
+
 			if(iscarbon(usr))
 				var/mob/living/carbon/C = usr
 				if(!C.get_active_hand())