nrf52840: right-size the SoftDevice RAM reservation (+8 KB heap arena) (#10903)

The app RAM ORIGIN in both nrf52840 linker scripts has been a hard-coded
0x20006000 (24 KB reserved for S140) since they were introduced, but
sd_ble_enable() with our fixed Bluefruit configuration (one peripheral
link, BANDWIDTH_MAX / ATT MTU 247, 0x1000 attribute table) requires
substantially less. The SoftDevice cannot use the gap and the app image
is linked above it, so every byte between the true requirement and the
ORIGIN is simply unusable RAM - on a part where 2.8.0 field reports
showed the heap arena at 99% use.

Lower the ORIGIN to 0x20004000 in both s140 v6 and v7 scripts. The heap
arena is the linker gap on this platform, so the change is worth exactly
+8,192 B of free heap on every nRF52840 board (verified: rak4631 .heap
section 124,572 -> 132,764 B; both v6- and v7-script boards link with
.data at 0x20004000).

Safety net: Bluefruit.begin()'s return value - previously discarded -
is now checked. If a future SoftDevice or Bluefruit config change raises
the requirement past the reservation, the node logs a critical error
instead of silently running without BLE, with instructions to re-measure
via CFG_DEBUG=1 ("SoftDevice's RAM requires: 0x...").

src/platform/nrf52/NRF52Bluetooth.cpp

@@ -4,6 +4,7 @@
 #include "HardwareRNG.h"
 #include "PowerFSM.h"
 #include "configuration.h"
+#include "error.h"
 #include "main.h"
 #include "mesh/PhoneAPI.h"
 #include "mesh/mesh-pb-constants.h"
@@ -275,7 +276,16 @@ void NRF52Bluetooth::setup()
     LOG_INFO("Init the Bluefruit nRF52 module");
     Bluefruit.autoConnLed(false);
     Bluefruit.configPrphBandwidth(BANDWIDTH_MAX);
-    Bluefruit.begin();
+    if (!Bluefruit.begin()) {
+        // sd_ble_enable() rejected our RAM base: the linker RAM ORIGIN
+        // (src/platform/nrf52/nrf52840_s140_v*.ld) is below what the SoftDevice needs for the
+        // current Bluefruit config. Without this check the node would silently run without BLE.
+        // Rebuild with -DCFG_DEBUG=1 to get "SoftDevice's RAM requires: 0x..." in the log, then
+        // raise the ORIGIN accordingly.
+        LOG_ERROR("Bluefruit.begin failed - SoftDevice RAM reservation too small for this config");
+        RECORD_CRITICALERROR(meshtastic_CriticalErrorCode_UNSPECIFIED);
+        return;
+    }
     // Clear existing data.
     Bluefruit.Advertising.stop();
     Bluefruit.Advertising.clearData();

src/platform/nrf52/nrf52840_s140_v6.ld

@@ -21,8 +21,17 @@ MEMORY
    * - Max ATT MTU
    * - Concurrent connection peripheral + central + secure links
    * - Event Len, HVN queue, Write CMD queue
+   *
+   * With our fixed Bluefruit config (1 peripheral link, BANDWIDTH_MAX / MTU 247,
+   * 0x1000 attr table) sd_ble_enable() reports a requirement well below the old
+   * 0x20006000 ORIGIN; every byte of gap is unusable RAM. 0x20004000 keeps a
+   * ~2+ KB margin over the reported base. NRF52Bluetooth::setup() now checks
+   * Bluefruit.begin() and records a critical error if the SoftDevice ever
+   * rejects this base (e.g. after an SD/Bluefruit config change) - re-measure
+   * with CFG_DEBUG=1 (Bluefruit logs "SoftDevice's RAM requires: 0x...")
+   * before raising bandwidth/MTU/link settings.
    */
-  RAM (rwx) :  ORIGIN = 0x20006000, LENGTH = 0x20040000 - 0x20006000
+  RAM (rwx) :  ORIGIN = 0x20004000, LENGTH = 0x20040000 - 0x20004000
 }
 
 SECTIONS

src/platform/nrf52/nrf52840_s140_v7.ld

@@ -18,8 +18,17 @@ MEMORY
    * - Max ATT MTU
    * - Concurrent connection peripheral + central + secure links
    * - Event Len, HVN queue, Write CMD queue
-   */ 
-  RAM (rwx) :  ORIGIN = 0x20006000, LENGTH = 0x20040000 - 0x20006000
+   *
+   * With our fixed Bluefruit config (1 peripheral link, BANDWIDTH_MAX / MTU 247,
+   * 0x1000 attr table) sd_ble_enable() reports a requirement well below the old
+   * 0x20006000 ORIGIN; every byte of gap is unusable RAM. 0x20004000 keeps a
+   * ~2+ KB margin over the reported base. NRF52Bluetooth::setup() now checks
+   * Bluefruit.begin() and records a critical error if the SoftDevice ever
+   * rejects this base (e.g. after an SD/Bluefruit config change) - re-measure
+   * with CFG_DEBUG=1 (Bluefruit logs "SoftDevice's RAM requires: 0x...")
+   * before raising bandwidth/MTU/link settings.
+   */
+  RAM (rwx) :  ORIGIN = 0x20004000, LENGTH = 0x20040000 - 0x20004000
 }
 
 SECTIONS