feat: Add offline mode support to FallbackNetwork

[joshwhiton]

Summary

• Adds local_files_only parameter to FallbackNetwork and G2P
• Respects TRANSFORMERS_OFFLINE and HF_HUB_OFFLINE env vars
• Enables fully offline operation when models are pre-cached

Problem

FallbackNetwork.from_pretrained() attempts network access even when TRANSFORMERS_OFFLINE=1 is set, because from_pretrained() does a HEAD request to check for model updates before falling back to cache.

Solution

Check the standard HuggingFace offline environment variables and pass local_files_only=True to from_pretrained() when they're set.

Use case

• Air-gapped deployments
• Offline applications
• Reducing startup latency when network is slow/unavailable

Test

import os
os.environ['TRANSFORMERS_OFFLINE'] = '1'
os.environ['HF_HUB_OFFLINE'] = '1'

from misaki import en
g2p = en.G2P()  # No network calls when model is cached

Notes

• Backwards compatible: default behavior unchanged when env vars aren't set
• Can also be set explicitly: G2P(local_files_only=True)