chore: batch RSR compliance — SPDX headers, SHA-pin actions, forbid(unsafe_code), CODE_OF_CONDUCT, CONTRIBUTING

- Add/fix SPDX-License-Identifier headers (AGPL→PMPL where needed)
- SHA-pin all GitHub Actions to commit hashes
- Add #![forbid(unsafe_code)] to safe Rust crates
- Add CODE_OF_CONDUCT.md (Contributor Covenant v2.1)
- Add CONTRIBUTING.md (standard template)

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

Author: hyperpolymath

.github/workflows/generator-generic-ossf-slsa3-publish.yml

@@ -66,7 +66,7 @@ jobs:
       actions: read   # To read the workflow path.
       id-token: write # To sign the provenance.
       contents: write # To add assets to a release.
-    uses: slsa-framework/slsa-github-generator/.github/workflows/generator_generic_slsa3.yml@v2.1.0
+    uses: slsa-framework/slsa-github-generator/.github/workflows/generator_generic_slsa3.yml@f7dd8c54c2067bafc12ca7a55595d5ee9b75204a # v2.1.0
     with:
       base64-subjects: "${{ needs.build.outputs.digests }}"
       upload-assets: true # Optional: Upload to a new release