test: make firmware logic bugs catchable without hardware (Workstream F)

The firmware has never been flashed, and a real bug already reached the
repo because of it: RD03E_FRAME_LEN was 5 for a 6-byte frame, so the footer
check collided with the distance high byte and EVERY distance reading was
garbage — always `lo | 0x5500`, about 218 metres, regardless of what the
sensor saw. That was pure logic with no hardware dependency. It should have
been catchable on a laptop, and there was simply no way to run the code.

Extracted the hardware-free logic out of the three drivers — rd03e_parse,
bmp280_compensate, mems_level — as moves rather than rewrites, carrying the
explanatory comments along with the code they explain. The drivers now own
only their bus I/O and call into the pure units, so nothing changes for the
real device.

`./run_tests.sh` builds them with gcc -Wall -Wextra -Werror plus a
dependency-free assert harness: 175 checks, 0 failed, from a clean tree.

Proven to catch the actual bug rather than assumed to: reintroducing
FRAME_LEN 5 fails four checks, including one that reads "a simple-report
frame is 6 bytes, not 5", plus the truncated-frame and 5-byte-window cases.
Restored, green again.

This does NOT make the firmware verified, and the README says so plainly —
it is called a narrow exception and scoped to pure logic. Wiring, timing,
real register behaviour and the reconstructed RD-03E frame format all still
need the physical board.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

firmware/esp32p4-sensor-node/main/CMakeLists.txt

@@ -1,5 +1,10 @@
 # Quantumancy sensor node — main component.
 #
+# The *_parse / *_compensate / *_level units are ESP-IDF-free pure logic,
+# split out of their drivers so they can also be compiled and tested on a
+# host with plain gcc — see ../test/run_tests.sh. They are listed here too
+# because the on-device build needs them linked in exactly the same way.
+#
 # device_config.h is intentionally NOT listed as a source: it's a header the
 # seeker generates locally (see device_config.h.example + README.md) and is
 # gitignored. If it's missing, the build will fail on the #include in
@@ -13,8 +18,11 @@ idf_component_register(
         "telemetry_client.c"
         "sensor_registry.c"
         "bmp280.c"
+        "bmp280_compensate.c"
         "rd03e.c"
+        "rd03e_parse.c"
         "mems_mic.c"
+        "mems_level.c"
     INCLUDE_DIRS
         "."
     REQUIRES