Normalize license headers to SPDX identifiers across repository

[arty-kk]

Motivation

• Replace verbose Apache-2.0 license blocks with concise SPDX tags to simplify headers and improve compatibility with license tooling.
• Keep file-level copyright attribution while making headers more uniform across sources, tests and scripts.
• Reduce repository noise from repeated multi-line license banners.

Description

• Replaced multi-line Apache License 2.0 headers with SPDX-FileCopyrightText and SPDX-License-Identifier: Apache-2.0 lines in numerous Python modules, test files, scripts and the C source fwht.c.
• Adjusted the build_proof_pack.py script to include the SPDX header and preserved the existing shebang ordering.
• No functional code logic changes were made; changes are limited to file header comments and minor comment formatting in the native C file.

Testing

• Ran the project's unit test suite with pytest; the test run completed successfully with all tests passing.

---

Codex Task