Fix remaining AGPL SPDX in Rust files and SHA-pin nested workflow actions

- AGPL-3.0-or-later → PMPL-1.0-or-later in .rs source files
- SHA-pin remaining checkout@v2/v3, upload-artifact@v3, cache@v3,
  setup-node@v3 in nested/submodule workflow files

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath