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/test/test_main.c

@@ -0,0 +1,23 @@
+// Host test runner for the ESP-IDF-free firmware logic units.
+// Exit status 0 = all checks passed, 1 = at least one failed.
+
+#include "test_util.h"
+
+int g_tests_run = 0;
+int g_tests_failed = 0;
+
+int main(void) {
+    printf("firmware host tests (pure logic only — no hardware involved)\n\n");
+
+    test_rd03e_parse();
+    test_bmp280_compensate();
+    test_mems_level();
+
+    printf("\n%d checks run, %d failed\n", g_tests_run, g_tests_failed);
+    if (g_tests_failed != 0) {
+        printf("FAILED\n");
+        return 1;
+    }
+    printf("OK\n");
+    return 0;
+}