Initial commit: he11lib mode-purity reconstruction library

Full implementation of Laguerre-Gauss modal reconstruction for gyrotron
beam diagnostics, per the approved design spec, plus tests, docs, and
a runnable end-to-end example.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

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+# he11lib
+
+Mode purity reconstruction of a free-space-propagating gyrotron RF beam from a
+set of thermal (flux) images taken at different distances from the output
+window.
+
+Decomposes the beam into Laguerre-Gauss (LG) modes referenced to a known
+waist size/location/wavelength, accounting for camera geometry, sensor noise,
+target thermal-diffusion blur, and unknown beam centering/pointing.
+
+See `docs/api.md` for the full API reference and
+`examples/full_pipeline_example.py` for a runnable end-to-end
+demonstration, and
+`docs/superpowers/specs/2026-07-02-gyrotron-mode-purity-design.md` for the
+full design.
+
+## Install (editable, for development)
+
+```bash
+pip install -e ".[dev]"
+```
+
+## Run tests
+
+```bash
+pytest
+```
+
+## License
+
+GPL-3.0-or-later. See `LICENSE`.
+
+## AI-assisted development disclosure
+
+The initial implementation of this library (design, code, tests, and
+documentation) was produced with AI assistance:
+
+- **Model:** Claude Sonnet 5 (`claude-sonnet-5`)
+- **Agent/harness:** Claude Code CLI, version 2.1.92
+
+All output was reviewed and directed by a human collaborator throughout
+development.