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

Adds SPDX-License-Identifier: MPL-2.0 header so GitHub Licensee detects a
recognised OSI licence. A PMPL-1.0-or-later preferred-licence notice
precedes the full MPL-2.0 fallback text. No source files changed.

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

Author: hyperpolymath