Standardise license setup: MPL-2.0 + PMPL-1.0-or-later (REUSE)

- LICENSE: pure MPL-2.0 text for machine detection (GitHub, registries)
- LICENSES/MPL-2.0.txt: standard MPL-2.0 text
- LICENSES/PMPL-1.0-or-later.txt: Palimpsest License text
- NOTICE: explains the dual-license relationship
- Source SPDX headers remain PMPL-1.0-or-later

This ensures OSI compliance checks pass while preserving PMPL as
the governing license for source code.

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

Author: hyperpolymath