Add heltec-v3_blemesh, the opt-in ESP32 BLE-mesh build, with the GATT mesh-peer

It rebuilds the host and controller for two connections and three
controller activities (the advertisement and both links); every other
ESP32 env keeps one connection and two activities. It inherits
heltec-v3's board_level = pr, so CI and MemBrowse measure it while #12010
is a draft, as they do rak4631_blemesh.

variants/esp32s3/heltec_v3/platformio.ini

@@ -17,3 +17,17 @@ build_flags =
   ${esp32s3_base.build_flags}
   -D HELTEC_V3
   -I variants/esp32s3/heltec_v3
+
+; The BLE-mesh opt-in build for heltec-v3. It carries the GATT mesh-peer transport, a second
+; peripheral link beside the phone's; network.enabled_protocols turns it on at runtime. The host
+; and controller are rebuilt for it: two connections, and three controller activities (the
+; advertisement and both links). Last key wins, so these follow the inherited values.
+[env:heltec-v3_blemesh]
+extends = env:heltec-v3
+build_flags = ${env:heltec-v3.build_flags}
+  -DHAS_BLE_GATT_MESH=1
+  -DBLE_GATT_MESH_ESP32_LINKS=2
+custom_sdkconfig =
+  ${esp32s3_base.custom_sdkconfig}
+  CONFIG_BT_NIMBLE_MAX_CONNECTIONS=2
+  CONFIG_BT_CTRL_BLE_MAX_ACT=3