🐛 Fix Fabric resource warning noise

[BoyAxl]

Summary

• Add explicit particle texture references to the standalone backpack models, reusing each model's existing texture alias.
• Remove Backpacked's stale Fabric refmap declarations now that the build does not generate or package backpacked.refmap.json.
• Leave unrelated dependency warnings from other mods untouched.

Rationale

This is intended to be a small resource/build cleanup for Fabric 26.1.2 startup noise. The model changes do not alter geometry, texture paths, rendering code, compatibility hooks, or gameplay behavior; they only provide the particle reference expected by the model baker. The refmap cleanup removes declarations for a file that is not produced by the current Fabric/Loom setup.

I intentionally avoided adding logs, fallbacks, helper abstractions, reflection, or compatibility reimplementations.

Testing

• Tested in a Minecraft 26.1.2 Fabric instance and confirmed the Backpacked model particle warnings no longer appear.
• Confirmed the Backpacked backpacked.refmap.json warnings no longer appear.
• Verified all assets/backpacked/models/backpacked/*.json files parse as valid JSON.
• Verified the built jar contains the generated backpack_dock recipe, advancement, and loot table data.
• Ran Fabric datagen and build locally. In my checkout, I used a temporary local Gradle flatDir to resolve unavailable dependency artifacts without modifying the repository Gradle files.

AI Assistance

This PR was prepared with AI assistance. I reviewed the resulting diff, kept the scope intentionally narrow, and tested the built jar locally before submitting. I take responsibility for the change and am happy to adjust it if there is a preferred project convention for these resources.