Bring the docs in line with both keypad fixes

AGENTS.md still carried a stale entry telling the reader to *lengthen* key
holds when a press seems ignored, which is the opposite of the fix and is
what broke the tooling in the first place. Replaced with the correction and
a pointer to the right section.

The keypad heading also claimed the hold time was the only cause. There were
two: the 2500 ms hold in key.py, and row_out missing volatile. Both are now
listed up front with a link to the detail.

Adds the regression test to the places someone would actually look: the
"How to run it" section in AGENTS.md, the layout listing, and a build step
in the README noting that a clean build is not evidence the keypad works,
since the -O2 dead-code elimination produces no warning.

README.md

@@ -69,6 +69,7 @@ keypresses silently stop working. Run the test after touching that code;
       calibration.bin        512-byte dump from a real radio
     docs/screenshots/        LCD captures used in this README
     tools/                   run, screenshot, inject keys, probe state
+      keypad_test.py         keypad regression test, boots its own instance
     harness/, stubs/, shim/, tests/   host build of the CW timing chain (stage A)
 
 ## Building
@@ -99,6 +100,14 @@ Needs a QEMU 7.2 source tree, `meson`, `ninja`, `libfdt-dev`, `libglib2.0-dev`,
     ./configure --target-list=arm-softmmu --disable-docs --disable-tools
     cd build && ninja qemu-system-arm
 
+Then check the build actually works, which takes about a minute:
+
+    python3 tools/keypad_test.py
+
+This matters more than it looks. The keypad can break silently under -O2 without
+any compiler warning -- see the `volatile` note in [Status](#status) -- so a clean
+build is not evidence that keypresses work.
+
 ## Running
 
     python3 tools/make_flash.py     # once, builds assets/flash.img