fix: make numba optional for base installs

[AkhilTrivediX]

Closes #4390

Summary

Moves numba out of the base dependency set and into the image/PDF dependency path. A plain installation no longer resolves llvmlite, while the partitioners that use XY-cut continue to receive numba through their existing extras.

Validation

• uv lock --locked
• Installed the project into a new base virtual environment and confirmed numba is absent
• Ran unstructured --help and partition_text in that clean base environment
• Confirmed uv export --locked omits numba, while uv export --locked --extra image includes it

[cubic-dev-ai]

No issues found across 2 files

<sub>Shadow auto-approve: would auto-approve. Moves numba from base to image/PDF extras, reducing unnecessary dependencies for plain installs.

Re-trigger cubic</sub>