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/Makefile

@@ -0,0 +1,40 @@
+# Host build for the ESP-IDF-free firmware logic units.
+#
+# Requires nothing but gcc and make. No ESP-IDF, no test framework, no
+# package manager. `make check` builds and runs; `run_tests.sh` wraps this
+# and is the entry point CI (and you) should call.
+
+CC      ?= gcc
+CFLAGS  ?= -std=c11 -O1 -g -Wall -Wextra -Werror
+LDLIBS  ?= -lm
+
+MAIN_DIR := ../main
+BUILD    := build
+
+# The pure units under test, moved out of their drivers precisely so they
+# can be compiled here without a cross-toolchain.
+UNITS := \
+	$(MAIN_DIR)/rd03e_parse.c \
+	$(MAIN_DIR)/bmp280_compensate.c \
+	$(MAIN_DIR)/mems_level.c
+
+TESTS := \
+	test_main.c \
+	test_rd03e_parse.c \
+	test_bmp280_compensate.c \
+	test_mems_level.c
+
+BIN := $(BUILD)/firmware_tests
+
+.PHONY: all check clean
+all: $(BIN)
+
+$(BIN): $(UNITS) $(TESTS) test_util.h $(MAIN_DIR)/rd03e_parse.h $(MAIN_DIR)/bmp280_compensate.h $(MAIN_DIR)/mems_level.h
+	@mkdir -p $(BUILD)
+	$(CC) $(CFLAGS) -o $@ $(UNITS) $(TESTS) $(LDLIBS)
+
+check: $(BIN)
+	./$(BIN)
+
+clean:
+	rm -rf $(BUILD)