IllegalArgumentException: No model for layer minecraft:player_slim#parcool.equipment_in

**ParCool version:** 4.0.0.2 (also reproduced with `ParCool-CompatibilityAddon` removed — happens with base ParCool alone)
**Minecraft:** 1.21.1, NeoForge 21.1.248
**Modpack context:** Large pack (~300 mods), including several other mods that also mixin into `PlayerRenderer`'s constructor (`entity_model_features`, `entity_texture_features`, `skinlayers3d`, `waveycapes`, `quickskin`, `grapplemod`, `create`, `tacztweaks`, `railways`, `do_a_barrel_roll`, `playeranimator`, `hammerlib`)

### Crash

```
java.lang.IllegalArgumentException: Failed to create player model for SLIM
at net.minecraft.client.renderer.entity.EntityRenderers.lambda$createPlayerRenderers$27(EntityRenderers.java:183)
...
Caused by: java.lang.IllegalArgumentException: No model for layer minecraft:player_slim#parcool.equipment_in
at net.minecraft.client.model.geom.EntityModelSet.bakeLayer(EntityModelSet.java:17)
at net.minecraft.client.renderer.entity.EntityRendererProvider$Context.bakeLayer(EntityRendererProvider.java:63)
at net.minecraft.client.renderer.entity.player.PlayerRenderer.handler$icg000$parcool$onInit(PlayerRenderer.java:5035)
at net.minecraft.client.renderer.entity.player.PlayerRenderer.(PlayerRenderer.java:67)
...
```

### What's happening

ParCool's `PlayerRendererMixin` bakes a `parcool.equipment_in` layer in its `onInit` hook during `PlayerRenderer` construction, but only for the regular (`player`) model — no corresponding `player_slim` layer definition was registered before the bake call, so constructing the slim-arm player renderer throws and crashes the whole client on startup.

### Reproducibility note

This isn't 100% consistent across mod combinations — in our testing, adding or removing an unrelated mod (one that never touches rendering) sometimes made the crash appear/disappear. Our best guess is that this is because ParCool's `onInit` mixin implicitly depends on state set up by one of the many *other* mods that also inject into `PlayerRenderer`'s constructor, and the apply order between them (determined by mod/mixin-config discovery order) shifts based on what else is installed.

If that's right, the real fix is registering the `player_slim` variant of the `equipment_in` layer alongside the regular one (or via the standard `RegisterLayerDefinitions`-style event) rather than relying on construction-time ordering relative to other mods' mixins — that would make it deterministic regardless of load order or what else is installed.

### To reproduce

Install ParCool 4.0.0.2 on 1.21.1/NeoForge with a player skin using the slim (Alex) arm model, alongside other mods that also inject into `PlayerRenderer`'s constructor (see mod list above). The crash occurs during initial resource load, before reaching the main menu.