Resprites boulder platforms and adds smoothing to them (#93588)

## About The Pull Request

Changes platforms to a more topdown perspective to be in line with lava
turfs and adds smoothing to them, ensuring that they always take up all
the space on their lava tile but don't go onto basalt

<img width="860" height="653" alt="dreamseeker_07DHZsrM1C"
src="https://github.com/user-attachments/assets/4522d3e4-27e0-4866-aa07-93d7eeb0f5e1"
/>

## Why It's Good For The Game
Currently large sections of platforms look a bit jank, as can be seen in
the original vent platform PR

<img width="923" height="784"
alt="489234293-9c222c28-f63a-4955-b4b4-c600e934c0c9"
src="https://github.com/user-attachments/assets/91b32618-2496-494e-ac3e-737d8ca999cc"
/>

This should make it much easier to discern which tiles are safe to step
on during hectic combat of vent defense

## Changelog
🆑
image: Boulder platforms spawned by throwing boulders on lava or
activating vents now have new sprites that smooth together
/🆑

code/game/objects/structures/lattice.dm

@@ -196,12 +196,11 @@
 /obj/structure/lattice/catwalk/boulder
 	name = "boulder platform"
 	desc = "A boulder, floating on the molten hot deadly lava. More like a BOATlder."
-	icon = 'icons/obj/ore.dmi'
-	icon_state = "boulder_platform"
+	icon = 'icons/obj/smooth_structures/boulder_platform.dmi'
+	icon_state = "boulder_platform-0"
 	base_icon_state = "boulder_platform"
-	smoothing_flags = NONE
-	smoothing_groups = null
-	canSmoothWith = null
+	smoothing_groups = SMOOTH_GROUP_BOULDER_PLATFORM
+	canSmoothWith = SMOOTH_GROUP_BOULDER_PLATFORM + SMOOTH_GROUP_FLOOR_LAVA
 	build_material = null
 	/// The type of particle to make before the platform collapses.
 	var/warning_particle = /particles/smoke/ash
@@ -224,10 +223,9 @@
 	. = ..()
 
 /obj/structure/lattice/catwalk/boulder/proc/pre_self_destruct()
-	if(istype(loc, /turf/open/lava/plasma))
-		add_overlay("plasma_cracks")
-	else
-		add_overlay("lava_cracks")
+	var/mutable_appearance/cracks_overlay = mutable_appearance('icons/obj/ore.dmi', istype(loc, /turf/open/lava/plasma) ? "plasma_cracks" : "lava_cracks", src)
+	cracks_overlay.blend_mode = BLEND_INSET_OVERLAY
+	add_overlay(cracks_overlay)
 	animate(src, alpha = 0, time = 2 SECONDS, pixel_y = -16, easing = QUAD_EASING|EASE_IN)
 	addtimer(CALLBACK(src, PROC_REF(self_destruct)), 2 SECONDS)