nrf52: fix loop-task stack overflow causing reset on BLE pairing/first-sync (#10944)

The Arduino loop task runs Meshtastic's entire cooperative OSThread
scheduler on the Adafruit core's stock 4 KB (1024-word) stack. The 2.8
first-sync path - AdminModule::handleSetConfig -> saveChanges ->
configChanged -> RadioInterface::reloadConfig -> LR11x0 reconfigure ->
SPI Lock::lock, with vsnprintf/USB-CDC logging frames stacked on top -
overflows it.

SWD fault capture on a T1000-E (pyocd vector_catch=h) proved it: the
loop task's SP was driven 40 bytes BELOW its own pxStack base, the
stack paint was consumed to the floor, and the crash was a BusFault in
xQueueSemaphoreTake dereferencing a semaphore handle that adjacent-heap
corruption had overwritten with log text (0x3f3f207c = "| ??"). The
core's HardFault handler is a bare NVIC_SystemReset, so in the field
this presents as a silent reboot ~seconds after a phone pairs (or a 30 s
zombie hang -> supervision timeout 0x8 when the corruption lands on task
TCBs instead). Also explains the set_time_only -> immediate-NodeInfo
crash variant reported on empty-DB nodes: same task, different deep
chain. 2.7.26 is unaffected because the deep frames (XEdDSA, satellite
map conversion, replay engine) did not exist.

Fix: set -DLOOP_STACK_SZ=2048 (words = 8 KB) for all nrf52840 targets.
Requires the #ifndef guard from meshtastic/Adafruit_nRF52_Arduino#7;
until that merges the flag is a harmless redefinition warning. Validated
on hardware: pairing + full sync completes, stack paint shows healthy
margin under load.

Also fix the boot-time reset-reason log: the core's init() caches
RESETREAS and W1C-clears the register before setup() runs, so the raw
read here has always printed 0. Use readResetReason() instead.
(preFSBegin's RESETREAS==0 gate is intentionally left as-is - its
degenerate GPREGRET-only behavior is what makes lfs recovery work.)

src/platform/nrf52/main-nrf52.cpp

@@ -378,7 +378,11 @@ void nrf52Setup()
     pinMode(ADC_V, INPUT);
 #endif
 
-    uint32_t why = NRF_POWER->RESETREAS;
+    // The Adafruit core's init() (cores/nRF5/wiring.c) caches RESETREAS into a static and then
+    // W1C-clears the hardware register before setup() ever runs, so a raw NRF_POWER->RESETREAS
+    // read here is ALWAYS 0. Use the core's cached copy so this log line is actually meaningful
+    // (0x1 pin reset, 0x2 watchdog, 0x4 soft reset/SREQ, 0x8 CPU lockup, 0x10000 System OFF wake).
+    uint32_t why = readResetReason();
     // per
     // https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.nrf52832.ps.v1.1%2Fpower.html
     LOG_DEBUG("Reset reason: 0x%x", why);

variants/nrf52840/nrf52.ini

@@ -23,6 +23,13 @@ build_flags =
   ${arduino_base.build_flags}
   -Wno-unused-variable
   -Isrc/platform/nrf52
+  ; The Arduino loop task runs the entire cooperative OSThread scheduler; the core's stock
+  ; 4 KB (1024-word) stack overflows during BLE first-sync (admin config-apply -> radio
+  ; reconfigure with XEdDSA/satmap/log frames stacked) - SWD-proven on T1000-E: SP driven
+  ; 40 B below pxStack, corrupting adjacent heap -> silent HardFault reset on phone pairing.
+  ; 2048 words = 8 KB, validated on hardware. Value is in WORDS; requires the #ifndef guard
+  ; from meshtastic/Adafruit_nRF52_Arduino#7 (harmless redefinition warning until it merges).
+  -DLOOP_STACK_SZ=2048
   -DLFS_NO_ASSERT                      ; Disable LFS assertions , see https://github.com/meshtastic/firmware/pull/3818
   -DMESHTASTIC_EXCLUDE_AUDIO=1
   -DMESHTASTIC_EXCLUDE_PAXCOUNTER=1