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/main.py

@@ -29,6 +29,7 @@ Setup:
 # ============================================================================
 
 import gc
+import os
 import time
 import network
 import ntptime
@@ -2713,4 +2714,5 @@ def main():
 # STARTUP
 # ============================================================================
 
-main()
+if os.environ.get("PRESTO_TEST_MODE") != "1":
+    main()