chore(licence): add MPL-2.0 fallback licence — PMPL-1.0-or-later preferred

Adds LICENSE file with SPDX-License-Identifier: MPL-2.0 so GitHub Licensee
detects a recognised OSI licence. The PMPL-1.0-or-later preferred-licence
notice and full MPL-2.0 fallback text are included.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: hyperpolymath