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>

pyproject.toml

@@ -0,0 +1,27 @@
+[build-system]
+requires = ["setuptools>=68", "wheel"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "he11lib"
+version = "0.1.0"
+description = "Mode purity reconstruction of free-space gyrotron beams from multi-plane thermal (flux) images."
+readme = "README.md"
+requires-python = ">=3.10"
+license = { text = "GPL-3.0-or-later" }
+dependencies = [
+    "numpy>=1.24",
+    "scipy>=1.10",
+    "matplotlib>=3.7",
+]
+
+[project.optional-dependencies]
+dev = [
+    "pytest>=7.4",
+]
+
+[tool.setuptools.packages.find]
+include = ["he11lib*"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]