Add SPDX header to LICENSE and set Cargo.toml license field

The governance/licence-consistency check requires an SPDX-License-Identifier
header on the LICENSE file and a `license` field in the manifest. The LICENSE
body is MPL-2.0 text, so stamp `SPDX-License-Identifier: MPL-2.0` (matching the
actual body) and set `license = "MPL-2.0"` (replacing `license-file`).

Verified with standards/scripts/check-licence-consistency.sh (passes).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DF9CcCuL4YJoqs26eHsYiA

Author: Jonathan D.A. Jewell

LICENSE

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