Restore license file for offline package builds - #337

Fixes #336.

The source repository no longer included the Apache-2.0 license file after the
license metadata migration. Package builds therefore relied on
coherent.licensed, which downloads the license text from the internet during
the build.

This restores the historical license file and removes the build plugin. With
Setuptools 77 or later, the source license file is included in both the sdist
and wheel license metadata without a network request from this plugin.

Validation:

• python -m pytest -q -p no:ruff (216 passed, 1 skipped)
• ruff check . using Ruff 0.12.0
• python -m build --no-isolation -q
• Verified that the generated sdist and wheel include LICENSE and do not
  reference coherent.licensed.
• git diff --check

I used OpenAI Codex to assist with investigating and preparing this change,
and reviewed the resulting code, packaging metadata, and artifacts before
submission.