Correct the docs: serial receive is implemented now

The previous commit documented serial receive as a permanent limitation, listing
what would be needed to build it. It has since been built, so that section was
actively misleading -- it told the reader not to try something that already works.

Replaced with what it takes to keep working, since each of the three pieces fails
silently on its own: USART1 needs a chardev, DMA must decrement CNDTR for USART
(serviced on the CNDTR read, which is where the driver looks), and DR writes must
reach the chardev and not just stderr. The last one is the trap -- transmit that
goes only to stderr is indistinguishable from the firmware ignoring the command.

README status table and build-verification steps updated to match.

README.md

@@ -40,7 +40,7 @@ has no public datasheet, so its driver is the only specification available.
 | Frequency entry | works, stored per band and kept |
 | Keypad and menu navigation | works, including waking from power save |
 | Serial output (firmware log) | works, appears in the web UI log |
-| Serial input / CPS programming | **not implemented**, see [AGENTS.md](AGENTS.md#what-this-cannot-do) |
+| Serial input, CPS programming protocol | works, `-serial` any chardev |
 | Timing accuracy | deliberately wrong, see [Timing](#timing) |
 | Radio/RF behaviour | not modelled |
 
@@ -77,6 +77,8 @@ keypresses silently stop working. Run the test after touching that code;
       keypad_test.py         keypad regression test, boots its own instance
       test_flash_persist.py  flash writes survive a power cycle
       test_freq_entry.py     a typed frequency takes effect and persists
+      test_serial_rx.py      the firmware answers programming commands
+      lib_kill_emulator.sh   cleanup that only ever kills emulators
       webui.py               web remote control: live LCD plus clickable keypad
       dn42_firewall.sh       restrict the web UI port to DN42 sources
       restore_flash.sh       roll the flash image back to its pristine state
@@ -118,6 +120,7 @@ Then check the build actually works, which takes about a minute:
     python3 tools/keypad_test.py
     python3 tools/test_flash_persist.py
     python3 tools/test_freq_entry.py
+    python3 tools/test_serial_rx.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