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.

.claude/hooks/quick_gate_hook.py

@@ -19,7 +19,7 @@ import subprocess
 import sys
 
 PROJECT_ROOT = Path(__file__).resolve().parents[2]
-WATCHED = re.compile(r"[\\/](?:protocol|tests)[\\/][^\\/]+\.py$")
+WATCHED = re.compile(r"[\\/](?:protocol|experiments|tests|tools)[\\/][^\\/]+\.py$")
 
 
 def main() -> int: