Add cross-platform CI test matrix + PyPI classifiers (#25)

* Add cross-platform CI test matrix, PyPI classifiers

CI: new test.yml runs lint (ubuntu) + a test matrix (ubuntu/macos/windows
x py3.10/3.12, core+dev, GPU tests skip) on push to main and PRs, closing the
gap where only the release publish.yml ran (ubuntu, no tests). Add PyPI
classifiers (OS/Python/topic). README Tests badge, CLAUDE.md CI note.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Make availability tests reflect installed deps, not assume gpu extra

The new core+dev CI matrix has no diffusers, so the invisible-engine
availability tests (asserting is_available() is True unconditionally) and the
two mocked invisible CLI tests (whose command gates on is_available before the
mock) failed. Assert availability == actual importability of torch+diffusers,
and patch the CLI availability gate so the mocked-engine tests run regardless of
the gpu extra.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/test.yml

@@ -0,0 +1,46 @@
+name: Tests
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+concurrency:
+  group: tests-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+      - uses: astral-sh/setup-uv@v7
+      - name: Sync dev environment
+        run: uv sync --frozen --extra dev
+      - name: Ruff lint
+        run: uv run ruff check
+      - name: Ruff format check
+        run: uv run ruff format --check
+
+  test:
+    name: test ${{ matrix.os }} py${{ matrix.python-version }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        # Cross-OS x Python-floor/recent. The default-install surface only
+        # needs the core deps plus dev tooling (no GPU extra): the model-running
+        # paths are availability-gated and skip when torch/diffusers are absent,
+        # so this matrix exercises metadata, identify, visible, and the cv2
+        # eraser on every OS without pulling the heavy ML stack.
+        os: [ubuntu-latest, macos-latest, windows-latest]
+        python-version: ["3.10", "3.12"]
+    steps:
+      - uses: actions/checkout@v6
+      - uses: astral-sh/setup-uv@v7
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Sync dev environment
+        run: uv sync --frozen --extra dev
+      - name: Run tests
+        run: uv run pytest -q