variants: enable OTA on Heltec T096 (nRF52840, shared rak4631_hw recipe)

The T096 is an nRF52840 with the same S140 v6 SoftDevice and flash geometry as
the RAK4631, so it can reuse the shared rak4631_hw OTA recipe (ENABLE_OTA +
OtaFlashLayout_nrf52.h — APP_BASE 0x26000, staging ceiling 0xD4000, all
static_asserts satisfied by the stock nrf52840_s140_v6[_extrafs] ldscripts).

With the OTAFIX bootloader (oltaco PR #42 adds the T096 board) a node applies
in place; without it the node still discovers/relays/serves OTA and merely
fetch-refuses apply — so a T096 is a useful OTA relay/seeder regardless.

MOTA_HW_ID='"Heltec_t096"' tags the EndF trailer so a .mota built for other
hardware is refused. Propagates to every T096 role via ${Heltec_t096.*}.

Verified: repeater + companion_radio_usb build clean, 14 OTA objects each,
EndF stamped (hw='Heltec_t096', v1.17.0).

variants/heltec_t096/platformio.ini

@@ -1,9 +1,14 @@
 [Heltec_t096]
-extends = nrf52_base
+; nRF52840 (same SoftDevice S140 v6 + flash geometry as the RAK4631), so it reuses the shared
+; rak4631_hw OTA recipe (ENABLE_OTA + OtaFlashLayout_nrf52.h). With the OTAFIX bootloader (oltaco
+; PR #42 adds the T096 board) it can apply in place; without it the node still discovers/relays/serves
+; OTA and just fetch-refuses apply (graceful) — a T096 is a valuable OTA relay/seeder either way.
+extends = rak4631_hw
 board = heltec_t096
 board_build.ldscript = boards/nrf52840_s140_v6.ld
-build_flags = ${nrf52_base.build_flags}
+build_flags = ${rak4631_hw.build_flags}
   ${sensor_base.build_flags}
+  -D MOTA_HW_ID='"Heltec_t096"'   ; OTA hardware tag (apply refuses a .mota built for other hardware)
   -I lib/nrf52/s140_nrf52_6.1.1_API/include
   -I lib/nrf52/s140_nrf52_6.1.1_API/include/nrf52
   -I variants/heltec_t096
@@ -42,14 +47,14 @@ build_flags = ${nrf52_base.build_flags}
   -D PIN_BAT_CTL=47
   -D DISPLAY_CLASS=ST7735Display
   -D DISPLAY_ROTATION=1
-build_src_filter = ${nrf52_base.build_src_filter}
+build_src_filter = ${rak4631_hw.build_src_filter}
   +<helpers/*.cpp>
   +<helpers/sensors>
   +<../variants/heltec_t096>
   +<helpers/ui/ST7735Display.cpp>
   +<helpers/ui/MomentaryButton.cpp>
 lib_deps =
-  ${nrf52_base.lib_deps}
+  ${rak4631_hw.lib_deps}
   ${sensor_base.lib_deps}
   adafruit/Adafruit ST7735 and ST7789 Library @ ^1.11.0
 debug_tool = jlink