chore(license): add Apache-2.0 and declare it in all six distributions

[eaitbrahim]

Closes #277

The repo is public but legally all-rights-reserved: no LICENSE file means nobody may
fork, modify, or contribute. This blocks every remaining open-source phase.

What

• LICENSE at the root: canonical Apache-2.0 (fetched from GitHub's license API, not
  retyped), appendix filled with Copyright 2026 CodeGate Software.
• license = "Apache-2.0" (PEP 639 SPDX form) in all six pyproject.tomls. Verified
  against a real build: both the root and a sub-package wheel now carry
  License-Expression: Apache-2.0 in METADATA — the field license scanners read, which
  the deprecated table form does not produce.
• CONTRIBUTING.md: why Apache-2.0 over AGPL-3.0 and MIT (patent grant + warranty
  disclaimer matter for software that moves money), recorded so the decision is
  challengeable in place.
• tests/test_licensing.py (written first, red, then green): canonical banner + END OF
  TERMS marker, filled appendix with no [yyyy] template left, SPDX licence in every
  distribution, decision recorded. Same repo-hygiene style as test_packaging.py.

Acceptance from #277

• LICENSE at repo root with the chosen text, copyright line, and year
• pyproject.toml declares the same licence (all six distributions)
• gh repo view --json licenseInfo reports it — verified post-merge (GitHub detects
  the file on main; will confirm and tick)
• Decision and reasoning recorded in CONTRIBUTING.md

Gates: pytest 2774 passed / 1 skipped, ruff check clean, mypy clean.

Until this merges: do not accept external pull requests — the issue's warning holds;
nothing changes for contributors until this is on main.