feat: showcase typed spans in Streamlit demo

[w3lld1]

Summary

• replace the demo's developer-local checkpoint with a fresh-clone-compatible v2 encoder cascade
• add a cached sidebar selector for typed and binary models
• color typed spans by taxonomy category and show category, subcategory, and confidence in tooltips
• add a category legend, per-span table, and focused renderer tests for typed, untyped, and multiple spans

Related issue

Fixes #57

Type of change

• Bug fix
• Feature
• Documentation
• Tests
• Refactor or maintenance

Testing

• python3 -m ruff format --check demo/streamlit_demo.py tests/test_demo_pytest.py
• python3 -m ruff check demo/streamlit_demo.py tests/test_demo_pytest.py
• python3 -m pytest tests/test_demo_pytest.py -v (3 passed)
• python3 -m py_compile demo/streamlit_demo.py tests/test_demo_pytest.py
• git diff --check
• pytest tests/test_inference_pytest.py -v -k "not TestAnswerStartToken" — collection was attempted but this runner does not have torch installed
• Interactive model-download smoke test — not run locally because the runner lacks Streamlit/model dependencies and has insufficient disk for the model stack

Checklist

• I kept the PR focused on one change.
• I added or updated tests/docs when needed.
• I checked that no secrets, API keys, or credentials are included.

Rights & sign-off (required)

• I certify that I have the right to submit this code and that it may be
  distributed under the repository's MIT license
  (see CONTRIBUTING).