docs: add explicit licenses for PoCs and specs

[oskarth]

Summary

• Adds explicit MIT, Apache-2.0, and CC0 license files.
• Clarifies code vs documentation licensing in README, including that file-level SPDX headers are authoritative.
• Replaces UNLICENSED SPDX markers in templates with MIT OR Apache-2.0.
• Updates clearly first-party private-bond custom-utxo script/test SPDX markers to MIT OR Apache-2.0.
• Leaves existing MIT, Apache-2.0, third-party, and generated dependency licenses unchanged.

Verification

• rg -n "UNLICENSED" --glob '!**/dependencies/**' --glob '!**/node_modules/**' . || true returns no matches.
• All Solidity files outside excluded dependency folders have SPDX headers.
• git diff --check passes.

[rymnc]

have all the contracts been updated?

[oskarth]

Yes. I checked the Solidity tree again.

All Solidity files outside dependency folders have SPDX headers, and there are no remaining UNLICENSED markers. I did not rewrite existing MIT or Apache-2.0 headers across all contracts, because those file-level SPDX headers are intentionally authoritative. The repo default now applies only where a file does not state a more specific license.

This PR updates the template UNLICENSED markers and the two clearly first-party custom-utxo script/test files that were still UNLICENSED. I also clarified that precedence in the README.