ci: add SPDX header to LICENSE + switch CodeQL language matrix Rust+actions

Governance / Licence consistency required an SPDX-License-Identifier
header on the LICENSE file's first few lines.

CodeQL was configured for javascript-typescript on a Rust repo and
failed with 'no source code seen'. Switched matrix to:
  - rust (build-mode: none)
  - actions (build-mode: none)

Both single-block additions; no semantic content change to LICENSE.

Author: hyperpolymath

LICENSE

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