fix(mcu): MCU-A2 — site-configurable mag declination, persisted in BKPSRAM

The magnetometer yaw correction used a hardcoded -0.61 deg literal baked
in for one deployment site. Yaw_Sensor was wrong by (site_decl + 0.61)
deg at every other site whenever the UM982 dual-antenna heading was
unavailable.

Backed the value with BKPSRAM (slots 1+2 — slot 0 is the MCU-A7
emergency flag) and exposed set_mag_declination_deg / get_mag_declination_deg.
Default returns the legacy -0.61 deg when no override has been written so
the original site stays correct out of the box; a host command (or
future GPS-derived auto-calibration) writes the new site value once and
it persists across every reset path until main-power removal.

Hardened with a +/-30 deg range clamp on both write AND read paths — real
magnetic declinations are roughly +/-25 deg worldwide, so a wider value
indicates a calibration error or BKPSRAM corruption (VBAT brown-out, bit
flip) rather than a legitimate site. Defensive read-side clamp prevents
a corrupted slot from propagating a wild heading offset.

Replaced the single use site at the magnetometer yaw computation with
the getter; legacy global Mag_Declination retained and kept in sync by
the setter for any external linkage.

Added test_mcu_a2_mag_declination (10 cases): default, set/get,
persistence across reset, power-cycle clear, write-side clamp both
directions, plausible-site passthrough, defensive read-side clamp on
corruption, wrong-magic fallback, pre-fix bearing-error regression.
MCU regression now 81/81.

9_Firmware/9_1_Microcontroller/tests/Makefile

@@ -70,6 +70,7 @@ TESTS_STANDALONE := test_bug12_pa_cal_loop_inverted \
                     test_mcu_a7_emergency_persist \
                     test_mcu_a5_pa_cal_gate \
                     test_mcu_a6_recovery_dispatch \
+                    test_mcu_a2_mag_declination \
                     test_gap3_iwdg_config \
                     test_gap3_temperature_max \
                     test_gap3_idq_periodic_reread \
@@ -175,6 +176,9 @@ test_mcu_a5_pa_cal_gate: test_mcu_a5_pa_cal_gate.c
 test_mcu_a6_recovery_dispatch: test_mcu_a6_recovery_dispatch.c
 	$(CC) $(CFLAGS) $< -o $@
 
+test_mcu_a2_mag_declination: test_mcu_a2_mag_declination.c
+	$(CC) $(CFLAGS) $< -lm -o $@
+
 # Gap-3 safety tests -- mock-only (needs spy log for GPIO sequence)
 test_gap3_emergency_stop_rails: test_gap3_emergency_stop_rails.c $(MOCK_OBJS)
 	$(CC) $(CFLAGS) $(INCLUDES) $< $(MOCK_OBJS) -o $@