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.

experiments/lab042_pluto_image_loopback.py

@@ -26,6 +26,7 @@ from __future__ import annotations
 
 import argparse
 import csv
+import functools
 import io
 import time
 from dataclasses import asdict, dataclass, field
@@ -193,6 +194,7 @@ class FunctionalTestResult:
 # ==================================================================== подготовка
 
 
+@functools.lru_cache(maxsize=4)
 def prepare_jpeg(
     source_path: Path = SOURCE_IMAGE,
     width: int = IMAGE_WIDTH,
@@ -207,6 +209,10 @@ def prepare_jpeg(
     заранее принятым значением, и возвращается вместе с байтами.
 
     Возвращает готовые байты JPEG в градациях серого и применённое качество.
+
+    Результат кэшируется: функция вызывается шестью функциональными
+    проверками подряд, а каждый вызов заново кодирует изображение в
+    нескольких вариантах качества.
     """
 
     if not source_path.exists():