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

@@ -64,7 +64,7 @@ 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@v1.4.0
+    uses: slsa-framework/slsa-github-generator/.github/workflows/generator_generic_slsa3.yml@68bad40844440577b33778c9f29077a3388838e9 # v1.4.0
     with:
       base64-subjects: "${{ needs.build.outputs.digests }}"
       upload-assets: true # Optional: Upload to a new release

.github/workflows/rust-ci.yml

@@ -51,6 +51,6 @@ jobs:
         run: cargo install cargo-tarpaulin
       - name: Generate coverage
         run: cargo tarpaulin --out Xml
-      - uses: codecov/codecov-action@v3
+      - uses: codecov/codecov-action@ab904c41d6ece82784817410c45d8b8c02684457 # v3
         with:
           files: cobertura.xml