test(py): add coverage for PyO3 bindings + gallery round-trip

[lostmygithubaccount]

Summary

Grows the Python test suite from 5 tests to 27. Covers the full dkdc_draw.core surface (new/save/load/export SVG/export PNG) plus error paths, and exercises every element type via the committed examples/gallery/*.draw.json fixtures.

Files

• `tests/test_core.py` (12 tests): unit tests per-function for the PyO3 surface + top-level re-export smoke test + error paths (nonexistent path, invalid JSON on save/svg/png).
• `tests/test_gallery_roundtrip.py` (15 tests, 3 × 5 parameterized): for each gallery drawing (flowchart, sticky, wireframe, sketch, patterns): save-reload fixed point, SVG well-formed, PNG magic bytes + nontrivial size.
• Deletes `tests/test_draw.py`: 4 of its 5 tests were narrower duplicates of the new test_core.py versions; its `HAS_NATIVE` ImportError guard was cargo-cult (the project ships via maturin and every test needs the native module). The unique smoke test (`test_import` / attr-hasattr loop) is preserved as `test_top_level_reexports_wrapper_api`.

How to run

```bash
bin/test-py # or
uv run pytest # or
bin/test # runs bin/test-rs + bin/test-py
```

Scope notes

Not added to `bin/check-py`. The existing convention is that `check-py` skips maturin+ty in CI (because `draw-app` needs GTK); adding pytest would need maturin too, so it'd be skipped in CI for the same reason. Keeping pytest on the `bin/test-py` path preserves the existing split: `check` = lint/type, `test` = execute.

`run_cli` is not covered — it's argv-driven and structural correctness is already covered by the Rust tests.

Test plan

• `bin/test` green (31 rust + 27 python = 58 tests, all pass)
• `bin/check` green

🤖 Generated with Claude Code