docs: adopt Apache License 2.0

[jackwener]

Summary

• add the canonical Apache License 2.0 text and project NOTICE
• declare the Apache-2.0 SPDX identifier in the root and every workspace package
• document the license in both READMEs and add a regression contract test

Validation

• node --test scripts/license-contract.test.mjs (3 passed)
• Apache LICENSE is byte-identical to the official LICENSE-2.0.txt
• package metadata JSON parse check
• git diff --check