IconForge: Antag and species icons, greyscale previews optimization (#94954)

## About The Pull Request

Converts species and antagonist icon generation to the batched
spritesheet system using IconForge, thanks to the new
`get_flat_uni_icon` implementation. Unfortunately the cost of *building*
the sprite is still expensive (GFI is always expensive, even a fancy
list-based one), but the generation is SIGNIFICANTLY faster. We will see
evidence of parity in the screenshot tests. but here:

<img width="892" height="634" alt="image"
src="https://github.com/user-attachments/assets/2a17f2e3-c024-41f6-9d1e-c2cb70642a81"
/>

The main advantage is that species and antag icons can now take
advantage of the development-time smart cache which invalidates
automatically. On the server this PR does very little except make antag
icon generation a little bit more likely to find and announce errors
(BYOND has a habit of silently eating weird icon proc calls).

Also optimizes the greyscale preview generator from #90940 (~2x speedup)
using `rustg_iconforge_generate_headless` instead of `Insert()` to build
the resulting sheets. This can be further optimized in the future by
implementing a smart cache, like batched spritesheets, and storing it in
the repo, but for now it's not important/slow enough to be worth the
effort. Also fixes a silent compilation error that would always happen
outside unit tests, but for some reason doesn't appear on local? Notice
how `map_icon_key` is not a defined variable anywhere. That's because
`USE_RUSTG_ICONFORGE_GAGS` is *never* defined at this point, so it was
always using the 'slow' generation.

I also took the liberty of cleaning up the cultist and heretic icon
generation randomly initializing a blade object when it could just use a
static access.

## Why It's Good For The Game

The subsystem timing may not be much faster, but the interactivity
benefits during spritesheet realization are undeniable. Opening the
preferences menu during init on local is orders of magnitude faster.

**Old**
Early Assets: 5.02 seconds
Greyscale Previews: 1.38 seconds

**Fresh (No Cache)**
Early Assets: 4.21 seconds
Greyscale Previews: 0.5 seconds

**Cache Invalidated**
Early Assets: 4.27 seconds

**Cache Hit**
Early Assets: 4.05~4.2 seconds

**Preferences lag:**
~6 sec to open to ~2 sec to open due to caching in dev

## Changelog

🆑
code: Optimized species and antagonist icon loading in the preferences
menu on local, speeding up time to open in development.
fix: GAGS map preview generation no longer silently errors outside of
unit tests due to a compilation error.
/🆑

code/controllers/subsystem/greyscale_previews.dm

@@ -1,8 +1,3 @@
-/// If we are in unit tests OR if we are not using iconforge, then we should make sure the icons we are using are valid.
-#if !defined(USE_RUSTG_ICONFORGE_GAGS) || defined(UNIT_TESTS)
-	#define CHECK_SPRITESHEET_ICON_VALIDITY
-#endif
-
 SUBSYSTEM_DEF(greyscale_previews)
 	name = "Greyscale Previews"
 	flags = SS_NO_FIRE
@@ -135,6 +130,11 @@ SUBSYSTEM_DEF(greyscale_previews)
 
 /datum/controller/subsystem/greyscale_previews/proc/ExportMapPreviewsForType(filename, list/entries)
 	var/list/icons = list()
+	#ifdef USE_RUSTG_ICONFORGE_GAGS
+	// make sure the error icon exists as a default
+	var/datum/universal_icon/empty_icon = uni_icon('icons/testing/greyscale_error.dmi', "")
+	icons[""] = empty_icon.to_list()
+	#endif
 
 	for (var/entry in entries)
 		var/atom/typepath
@@ -162,25 +162,46 @@ SUBSYSTEM_DEF(greyscale_previews)
 		// This is what the actual icon state will be in the map_icon .dmi
 		var/key = reskin_icon_state ? "[typepath]--[icon_state]" : "[typepath]"
 
-	#ifdef CHECK_SPRITESHEET_ICON_VALIDITY
+	#ifdef USE_RUSTG_ICONFORGE_GAGS
+		// No large icons, use icon_preview and icon_preview_state instead.
+		var/datum/universal_icon/map_icon = SSgreyscale.GetColoredIconByTypeUniversalIcon(greyscale_config, greyscale_colors, icon_state)
+		icons[key] = map_icon.to_list()
+	#else
 		var/icon/map_icon = icon(SSgreyscale.GetColoredIconByType(greyscale_config, greyscale_colors)) // No large icons, use icon_preview and icon_preview_state instead.
+	#ifdef UNIT_TESTS // this is much slower using legacy generation, so limit it to unit tests.
 		if (map_icon.Width() > 32 || map_icon.Height() > 32)
 			stack_trace("GAGS configuration is trying to generate a map preview graphic for '[typepath]' (icon state: [icon_state]), which has a large icon. This is not suppoorted; implement icon_preview instead.")
 			continue
 		if (!(icon_state in map_icon.IconStates()))
 			stack_trace("GAGS configuration missing icon state ([icon_state]) needed to generate map preview graphic for '[typepath]'. Make sure the right greyscale_config is set up.")
 			continue
+	#endif
 		map_icon = icon(map_icon, icon_state)
 		icons[key] = map_icon
-	#else // will be updated to use iconforge's new .dmi spritesheet generation instead
-		var/icon/map_icon = icon(SSgreyscale.GetColoredIconByType(greyscale_config, greyscale_colors))
-		map_icon = icon(map_icon, icon_state)
-		icons[map_icon_key] = map_icon
 	#endif
 
+	#ifdef USE_RUSTG_ICONFORGE_GAGS
+	// generate all previews in parallel
+	var/holder = "tmp/greyscale_previews.dmi"
+	var/list/headless_result = rustg_iconforge_generate_headless(holder, json_encode(icons), FALSE)
+	if(!istype(headless_result))
+		stack_trace("Could not generate greyscale previews using rustg_iconforge_generate_headless! The output format was invalid (JSON did not decode to a list). Got: [headless_result]")
+	if(!length(headless_result))
+		stack_trace("Could not generate greyscale previews using rustg_iconforge_generate_headless! The output list was empty. Got: [json_encode(headless_result)]")
+	if(!isnull(headless_result["error"]))
+		stack_trace("Could not generate greyscale previews using rustg_iconforge_generate_headless! There was an error: [headless_result["error"]]")
+	if(headless_result["file_path"] != holder)
+		stack_trace("Could not generate greyscale previews using rustg_iconforge_generate_headless! The output file_path differed from the input. Got: [json_encode(headless_result)]")
+	// testing size is cheap with rustg, so no need to limit to unit tests.
+	if(headless_result["width"] != 32 || headless_result["height"] != 32)
+		stack_trace("Could not generate greyscale previews using rustg_iconforge_generate_headless! The output size was not 32x32. Got: [json_encode(headless_result)]")
+	if(!fexists(holder))
+		stack_trace("Could not generate greyscale previews using rustg_iconforge_generate_headless! The output file does not exist on the filesystem! Got: [json_encode(headless_result)]")
+	#else
 	var/icon/holder = icon('icons/testing/greyscale_error.dmi')
 	for(var/state in icons)
 		holder.Insert(icons[state], state)
+	#endif
 
 	var/filepath = "icons/map_icons/[filename].dmi"
 #ifdef UNIT_TESTS
@@ -192,7 +213,3 @@ SUBSYSTEM_DEF(greyscale_previews)
 	if(old_md5 != new_md5)
 		stack_trace("Generated map icons were different than what is currently saved. If you see this in a CI run it means you need to run the game once through initialization and commit the resulting files in 'icons/map_icons/'")
 #endif
-
-#ifdef CHECK_SPRITESHEET_ICON_VALIDITY
-	#undef CHECK_SPRITESHEET_ICON_VALIDITY
-#endif

code/controllers/subsystem/processing/greyscale.dm

@@ -1,8 +1,3 @@
-/// Disable to use builtin DM-based generation.
-/// IconForge is 250x times faster but requires storing the icons in tmp/ and may result in higher asset transport.
-/// Note that the builtin GAGS editor still uses the 'legacy' generation to allow for debugging.
-#define USE_RUSTG_ICONFORGE_GAGS
-
 PROCESSING_SUBSYSTEM_DEF(greyscale)
 	name = "Greyscale"
 	flags = SS_BACKGROUND