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/scorecard.yml

@@ -27,6 +27,6 @@ jobs:
           results_format: sarif
 
       - name: Upload results
-        uses: github/codeql-action/upload-sarif@v4
+        uses: github/codeql-action/upload-sarif@6624720a57d4c312633c7b953db2f2da5bcb4c3a # v3
         with:
           sarif_file: results.sarif