Move docstring copyright notices to SPDX header comments

[mcdigman]

Summary

Pull the Copyright 2026 Matthew C. Digman line out of each module docstring and replace it with a two-line SPDX header at the top of the file:

# SPDX-FileCopyrightText: Copyright 2026 Matthew C. Digman
# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

Applied to the 9 project files that had a copyright notice in their docstring: tests/test_pyakima.py, docs/conf.py, pyakima/__init__.py, pyakima/pyakima.py, and all five pyakima/demos/*.py.

Why this exact form

• Keeping the Copyright prefix on the SPDX-FileCopyrightText value means the notice still matches ruff's flake8-copyright (CPY001) default notice-rgx, which requires the literal word Copyright followed by whitespace and a year. The bare SPDX-FileCopyrightText: 2026 … form fails that regex.
• SPDX-FileCopyrightText: Copyright <year> <author> is still valid SPDX/REUSE (the value is free-form text).
• The license identifier matches the PEP 639 expression Apache-2.0 WITH LLVM-exception declared in pyproject.toml.

Left untouched

• docs/conf.py copyright = '2025-2026, Matthew C. Digman' — the Sphinx-mandated config variable (keeps its # noqa: A001), not a docstring notice.
• Small stub files with no existing notice (__init__.py stubs, _version.py) — nothing to move; they're under ruff's min-file-size = 1024.

Verification

• ruff check / ruff format --check — pass (also all pre-commit hooks passed on commit)
• ruff check --preview --select CPY001 — pass
• py_compile on all changed files — OK

🤖 Generated with Claude Code