chore(license): migrate MPL-2.0 (no SPDX header) → MPL-2.0 (solo) (#23)

Replace LICENSE file with canonical MPL-2.0 text and update package
metadata (Cargo.toml/package.json/deno.json/mix.exs/pyproject.toml where
present) to declare the new SPDX identifier.

Rationale: solo.

Part of estate-wide metadata cleanup 2026-05-26.

Author: hyperpolymath