fix(mcu): MCU-A4 — BKPSRAM warm-restart bypass for OCXO 180 s warmup

Every boot waited the full 180 s OCXO warmup soak — even an
IWDG/SYSRESETREQ reset that takes seconds and leaves the OCXO oven hot
lost three minutes of bringup time.

Added BKPSRAM slot 3 (magic 0xCA1C1F1E) with warmup_persist_set/check
helpers next to the existing MCU-A2/A7 BKPSRAM block. Cold-boot path
now arms the flag at the end of the full 180 s soak; subsequent boots
that find the flag still set know the OCXO oven is still hot and the
crystal is settled, so they wait 5 s and move on. Power-cycle clears
BKPSRAM and forces the full soak again — safe default, operator can't
accidentally skip the warmup by yanking and re-applying power.

Added test_mcu_a4_ocxo_warm_restart (7 cases): cold boot soaks 180 s
and sets the flag; warm reset is 5 s; 5 consecutive warm resets stay
fast; power-cycle restores the cold path; cold-after-power-cycle
re-arms the bypass; pre-fix regression confirms 10 warm restarts save
1750 s vs the old always-180-s path. MCU regression now 82/82.

9_Firmware/9_1_Microcontroller/tests/Makefile

@@ -71,6 +71,7 @@ TESTS_STANDALONE := test_bug12_pa_cal_loop_inverted \
                     test_mcu_a5_pa_cal_gate \
                     test_mcu_a6_recovery_dispatch \
                     test_mcu_a2_mag_declination \
+                    test_mcu_a4_ocxo_warm_restart \
                     test_gap3_iwdg_config \
                     test_gap3_temperature_max \
                     test_gap3_idq_periodic_reread \
@@ -179,6 +180,9 @@ test_mcu_a6_recovery_dispatch: test_mcu_a6_recovery_dispatch.c
 test_mcu_a2_mag_declination: test_mcu_a2_mag_declination.c
 	$(CC) $(CFLAGS) $< -lm -o $@
 
+test_mcu_a4_ocxo_warm_restart: test_mcu_a4_ocxo_warm_restart.c
+	$(CC) $(CFLAGS) $< -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 $@