Run the core tests from the gate and finish the layout rename

The gate now runs pytest as well, so one command answers whether the code
base is broken: syntax, protocol imports, the 125 core tests, then the lab
functional suites. It fails when a test fails, verified by feeding it a
deliberately broken test.

Two leftovers from moving the labs into experiments/ are fixed: the lab
command still pointed at tests/, and the hook only watched protocol and
tests, so edits under experiments/ and tools/ triggered nothing.

Profiling the 3.6 s gate: 0.24 s syntax, 0.12 s imports, 0.61 s pytest,
2.7 s lab functional suites, of which Lab042's software loopback is 1.14 s
because it pushes a whole image through the chain. The pytest addition is
the small part. prepare_jpeg is now cached, since six checks call it and
each call re-encoded the image at several qualities.

README described the gate without the tests it now runs.

README.md

@@ -98,7 +98,7 @@ python -m pip install -r requirements-hardware.txt
 python -m pytest tests/ -q
 ```
 
-**Дешёвый шлюз** — синтаксис, импорт `protocol`, функциональные проверки лабораторных:
+**Дешёвый шлюз** — синтаксис, импорт `protocol`, быстрые проверки ядра и функциональные проверки лабораторных. Отвечает на один вопрос: не сломана ли кодовая база прямо сейчас:
 
 ```bash
 python tools/quick_gate.py