Revamp CI workflow structure and job matrix

[srichs]

Motivation

• Separate linting, type-checking, and testing to make failures easier to diagnose and reduce noisy runs.
• Add workflow concurrency with cancellation and narrow workflow permissions to contents: read for safer run behavior.
• Expand test coverage to multiple OS/Python axes so CI better reflects supported environments.

Description

• Replaced the single tests job with three jobs: lint (runs ruff, black, isort), type-check (runs mypy), and a matrix test job (runs pytest), updating steps to use a common checkout/setup/install pattern in .github/workflows/ci.yml.
• Added workflow-level concurrency to cancel-in-progress runs on the same ref and restricted permissions to contents: read in the workflow file.
• Introduced a test matrix across os: [ubuntu-latest, macos-latest, windows-latest] and python-version: ["3.11", "3.12"] with fail-fast: false and standardized dependency installation via pip install -r requirements-dev.txt.
• Kept existing test invocation and coverage enforcement as pytest --cov=veracrypt --cov-report=term-missing --cov-fail-under=80.

Testing

• Ran the project test suite locally with pytest -q, which succeeded with 42 passed tests.

---

Codex Task