Disposal tweaks (#22999)

The disposal "mainline," the main trunk that wraps around the whole
ship, has been painted with a yellow stripe for better distinction with
both mapping and in-game diagnostics. Relevant code has been changed (a
microscopic amount) to make the stuff actually work, too.

The yellow stuff's currently not constructable due to the colossal
amount of additional codework to make them such for minimal return.

This PR additionally makes packages able to be mailed to the machinist's
workshop properly; due to a mapping error (very easy to miss) people
weren't able to do so before.

(CL)
* Painted the 'mainline' disposal pipes yellow for better distinction
with mapping.
* Adjusted some disposal piping to be tidier.
* Packages can now once again be mailed to the Machinist's Workshop.

## Images
<img width="1274" height="780" alt="image"
src="https://github.com/user-attachments/assets/51374227-f3e7-42ca-8a13-64d278ec0d0b"
/>
<img width="1343" height="1119" alt="image"
src="https://github.com/user-attachments/assets/4ae08fe7-ce09-44a9-8158-99dece757b47"
/>
<img width="1175" height="1092" alt="image"
src="https://github.com/user-attachments/assets/5a1b3aa4-1e51-4bc1-b1f4-2f341cef8ca1"
/>
<img width="449" height="594" alt="image"
src="https://github.com/user-attachments/assets/ba44fb77-8c81-41fc-b07f-a3593e063e41"
/>

code/modules/recycling/disposal.dm

@@ -1084,9 +1084,9 @@
 /obj/structure/disposalpipe/proc/welded()
 	var/obj/structure/disposalconstruct/C = new (src.loc)
 	switch(icon_state)
-		if("pipe-s")
+		if("pipe-s", "pipe-s-yellow", "pipe-s-blue")
 			C.ptype = 0
-		if("pipe-c")
+		if("pipe-c", "pipe-c-yellow", "pipe-c-blue")
 			C.ptype = 1
 		if("pipe-j1")
 			C.ptype = 2
@@ -1135,13 +1135,23 @@
 
 /obj/structure/disposalpipe/segment/Initialize()
 	. = ..()
-	if(icon_state == "pipe-s")
+	if(icon_state == "pipe-s" || icon_state == "pipe-s-yellow" || icon_state == "pipe-s-blue")
 		dpdir = dir | turn(dir, 180)
 	else
 		dpdir = dir | turn(dir, -90)
 
 	update()
 
+/obj/structure/disposalpipe/segment/mainline
+	name = "mainline disposal pipe"
+	desc = "A disposal pipe indicated by a yellow stripe to be part of the main trunk loop."
+	icon_state = "pipe-s-yellow"
+
+/obj/structure/disposalpipe/segment/blue
+	name = "disposal pipe"
+	desc = "A disposal pipe marked by a blue stripe."
+	icon_state = "pipe-s-blue"
+
 /// Z-Level stuff
 /obj/structure/disposalpipe/up
 	icon_state = "pipe-u"