governance: LICENSE missing SPDX-License-Identifier header

[hyperpolymath]

Split out from the 2026-06-13 estate checkpoint (hyperpolymath/echidna#238).

governance / Licence consistency fails on every PR:

[OK] LICENSE file found: LICENSE
[ERROR] LICENSE file has no 'SPDX-License-Identifier:' header on its first few lines.
[OK] Manifest licence (Cargo.toml): MPL-2.0
[ERROR] Licence consistency check failed.

(standards/scripts/check-licence-consistency.sh). Pre-existing.

Fix is owner-only per estate policy (#216 / #83 — license/SPDX edits are manual, owner-only): add SPDX-License-Identifier: MPL-2.0 near the top of LICENSE, keeping LICENSE and LICENSE.txt identical.