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>

.gitignore

@@ -0,0 +1,23 @@
+# Python
+__pycache__/
+*.py[cod]
+*.egg-info/
+.eggs/
+build/
+dist/
+
+# Virtual environments
+.venv/
+venv/
+
+# Test / tool caches
+.pytest_cache/
+.mypy_cache/
+.ruff_cache/
+
+# Editors
+.vscode/
+.idea/
+
+# Claude Code local settings (machine-specific, not project config)
+.claude/settings.local.json