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/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