chore(licence): normalise to MPL-2.0 + CC-BY-SA-4.0 (canonical pair) (#37)

Estate licence normalisation. LICENSES/={MPL-2.0,CC-BY-SA-4.0}; root
LICENSE=verbatim MPL-2.0 (GitHub display); code->MPL-2.0,
docs->CC-BY-SA-4.0; vendored untouched. Residual mentions (mostly
legitimate anti-AGPL rules/deny-lists): 5. Manual-review licence PR.

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
 ==================================