ci: add cross-platform test matrix (ubuntu, macos, windows × py3.11, py3.12)

[Gradata]

Problem

.github/workflows/sdk-publish.yml only runs on v* tag push, and it only runs the build — it does not run pytest. There is no test CI on push or PR at all. That means:

• Regressions land on main with no machine-verified signal.
• Platform-specific code paths (fcntl on POSIX, msvcrt on Windows) are never exercised outside a single developer's local box.
• macOS parity is assumed via code-path equivalence with Linux, not verified.

Proposed

Add .github/workflows/test.yml that runs on:

• push to main
• pull_request targeting main

Matrix: { os: [ubuntu-latest, macos-latest, windows-latest], python: [\"3.11\", \"3.12\"] } — 6 jobs.

Each job:

1. Checkout
2. Setup Python
3. Install uv
4. cd Gradata && uv sync --all-extras --dev
5. uv run pytest tests/ -q

Acceptance

• Workflow exists and is green on a PR touching src/gradata/
• Windows and macOS skip counts are documented (expected: 2 POSIX on Windows, 2 msvcrt on macOS/Linux)
• Failure on any OS blocks merge (branch protection follow-up, out of scope for this PR)

Out of scope

• Coverage gates
• Lint / type check jobs (separate workflow)
• Branch protection rule changes (repo settings)