chore(licence): normalise to MPL-2.0 (code) + CC-BY-SA-4.0 (docs) (#48)

- LICENSES/ = {MPL-2.0.txt, CC-BY-SA-4.0.txt}; root LICENSE = verbatim MPL-2.0 (GitHub display)
- SPDX: code -> MPL-2.0, docs (.md/.adoc) -> CC-BY-SA-4.0; metadata/badge fixes; vendored untouched

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

LICENSE

@@ -1,6 +1,3 @@
-SPDX-License-Identifier: MPL-2.0
-SPDX-FileCopyrightText: 2024-2026 Jonathan D.A. Jewell (hyperpolymath)
-
 Mozilla Public License Version 2.0
 ==================================