fix(LICENSE): restore SPDX-License-Identifier header

The estate-standardization commit 4afd789 inadvertently stripped the
leading `SPDX-License-Identifier: MPL-2.0` line from LICENSE. The
standards governance/Licence-consistency check requires this header on
the first few lines of LICENSE so downstream scanners (REUSE,
cargo-license, etc.) can identify the licence.

Restoring prior state — not a licensing change. Owner-authorised
restore (2026-06-11) after CI revealed the regression.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

LICENSE

@@ -1,3 +1,5 @@
+SPDX-License-Identifier: MPL-2.0
+
 Mozilla Public License Version 2.0
 ==================================