ML-184: add Presto smoke test harness

Add a headless smoke-test harness for the Presto microPython app using
pimoroni-emulator mock modules.  A PRESTO_TEST_MODE=1 guard prevents
main() from running at import time so draw functions can be called
directly from tests.

- presto/main.py: wrap main() with PRESTO_TEST_MODE guard
- presto/tests/: conftest with emulator fixtures, 7 smoke tests
- presto/mise.toml: add [tasks.test]
- presto/README.md, presto/AGENTS.md: add Testing section
- .gitignore: exclude test __pycache__, pytest_cache, fixture pngs

presto/tests/conftest.py

@@ -0,0 +1,134 @@
+"""Presto smoke test configuration and helpers.
+
+Uses the pimoroni-emulator's mock system so ``main.py`` can be imported
+on CPython without real MicroPython hardware.  The emulator's headless
+display is set up as a session-scoped fixture; ``main`` is imported once
+after the mocks are in place.
+"""
+
+import os
+import sys
+from pathlib import Path
+
+import pytest
+
+# Ensure the presto directory is on sys.path so `import main` works.
+_presto_dir = Path(__file__).resolve().parent.parent
+if str(_presto_dir) not in sys.path:
+    sys.path.insert(0, str(_presto_dir))
+
+# Prevent main() from running when we import the module.
+os.environ["PRESTO_TEST_MODE"] = "1"
+
+
+# ---------------------------------------------------------------------------
+# Emulator session setup
+# ---------------------------------------------------------------------------
+
+@pytest.fixture(scope="session")
+def _emulator():
+    """Install mock MicroPython modules and create a headless Presto display.
+
+    This must run before ``main`` is imported because ``main.py`` imports
+    ``presto``, ``network``, ``picographics`` etc. at module level.
+    """
+    from emulator import _emulator_state
+    from emulator.devices import get_device
+    from emulator.display import create_display
+    from emulator.mocks import install_mocks
+
+    device = get_device("presto")
+    _emulator_state["device"] = device
+    _emulator_state["running"] = True
+    _emulator_state["headless"] = True
+
+    install_mocks()
+
+    display = create_display(device, headless=True)
+    _emulator_state["display"] = display
+    display.init()
+
+    yield
+
+    _emulator_state["running"] = False
+    display.close()
+
+
+# ---------------------------------------------------------------------------
+# Module fixture — import main once
+# ---------------------------------------------------------------------------
+
+@pytest.fixture(scope="session")
+def main_module(_emulator):
+    """Import ``main.py`` after the emulator mocks are installed.
+
+    The ``PRESTO_TEST_MODE=1`` guard ensures ``main()`` does not run.
+    ``fetch_thumbnail`` is monkey-patched to raise on any network call.
+    """
+    import main
+
+    # Guard against accidental network access in smoke tests.
+    def _raise_on_network(*_args, **_kwargs):
+        raise RuntimeError("Network call in smoke test — use mock data")
+
+    main.fetch_thumbnail = _raise_on_network
+
+    return main
+
+
+# ---------------------------------------------------------------------------
+# Per-class display initialisation
+# ---------------------------------------------------------------------------
+
+@pytest.fixture(scope="class")
+def init_display(main_module):
+    """Call ``init_display()`` once per test class so pens are ready."""
+    main_module.init_display()
+
+
+# ---------------------------------------------------------------------------
+# Mock record helper (plain function — no main import needed)
+# ---------------------------------------------------------------------------
+
+def make_mock_record(index=0):
+    """Return a fully-populated mock record dict for smoke tests.
+
+    Every key required by draw functions is pre-populated.  Thumbnail
+    cache entries are set to ``None`` / ``True`` so the renderer uses
+    grey placeholders and never calls ``fetch_thumbnail``.
+    """
+    title = f"Mock Album {index}"
+    artist = f"Mock Artist {index}"
+    return {
+        # API-level fields
+        "id": f"rec-{index}",
+        "selected_release_id": f"release-uuid-{index}",
+        "title": title,
+        "artists": [artist],
+        "format": "Vinyl",
+        "release_date": "1959-08-17",
+        "genres": ["Jazz", "Modal"],
+        "record_type": "LP",
+        "purchased_at": "2024-03-15",
+        # Pre-computed display strings (row view)
+        "_display_title": title,
+        "_display_artists": artist,
+        "_display_meta": "Vinyl | 1959",
+        "_display_title_lines": [(title, 120)],
+        "_display_artist_lines": [(artist, 120)],
+        "_display_meta_lines": [("Vinyl | 1959", 120)],
+        # Thumbnail cache (row view) – force placeholder
+        "_thumb_url": "",
+        "_thumb_data": None,
+        "_thumb_failed": True,
+        # Detail-view cache keys
+        "_detail_thumb_data": None,
+        "_detail_thumb_failed": True,
+        "_detail_title_lines": [(title, 22)],
+        "_detail_artist_lines": [(artist, 15)],
+        "_detail_genre_lines": [("Jazz, Modal", 15)],
+        "_detail_meta_lines": [("LP | Vinyl | 1959", 15)],
+        "_detail_purchased_lines": [("Purchased: 2024-03-15", 15)],
+        # Row height (computed by prepare_record_list)
+        "_row_height": 88,
+    }