chore(solo): move PR #31's new ProMicro solo env into solo/promicro/

Merged after the fact, so it followed the old convention (solo env inside
variants/promicro/platformio.ini alongside the shared board configs). Moved
to match every other board post-restructuring -- extends still points at
Promicro, defined in variants/promicro/platformio.ini.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

solo/promicro/platformio.ini

@@ -0,0 +1,45 @@
+; Solo build for ProMicro (nRF52840) -- extends the board base defined in
+; variants/promicro/platformio.ini. Solo configs live in their own top-level
+; solo/ folder, separate from the shared upstream board variants in variants/,
+; so this repo's Solo-specific additions don't get mixed in with configs
+; other MeshCore forks/upstream also carry.
+;
+; Full on-device UI, with CardKB support: CardKB shares the board's primary
+; I2C bus (already used by the OLED display / RTC) instead of a dedicated
+; second bus -- the default Wire1 pins on this variant (D13/D14) physically
+; collide with the LoRa SPI bus (P_LORA_NSS=13, P_LORA_MOSI=14), and no other
+; free GPIO pair is broken out on this board for a second dedicated bus. Do
+; NOT define ENV_PIN_SDA/ENV_PIN_SCL here -- that would bring up a real Wire1
+; on the colliding pins instead.
+[env:ProMicro_companion_solo_dual]
+extends = Promicro
+board_build.ldscript = boards/nrf52840_s140_v6_extrafs.ld
+board_upload.maximum_size = 712704
+; -Ofast (the core's default) doesn't fit a feature-complete solo build;
+; -Os fits with headroom.
+build_unflags = -Ofast
+build_flags = ${Promicro.build_flags}
+  -I examples/companion_radio/ui-new
+  -D MAX_CONTACTS=350
+  -D MAX_GROUP_CHANNELS=40
+  -D BLE_PIN_CODE=123456
+  -D DUAL_SERIAL=1
+  -D OFFLINE_QUEUE_SIZE=256
+  -D DISPLAY_CLASS=SSD1306Display
+  -D FIRMWARE_SOLO_BUILD=1
+  -D UI_SENSORS_PAGE=1
+  -D ENABLE_SCREENSHOT
+  -Os
+  ; CardKB on the shared primary bus (D8=SDA / D7=SCL on this board).
+  -D CARDKB_I2C=Wire
+;  -D MESH_PACKET_LOGGING=1
+;  -D MESH_DEBUG=1
+build_src_filter = ${Promicro.build_src_filter}
+  +<helpers/ui/SSD1306Display.cpp>
+  +<helpers/ui/MomentaryButton.cpp>
+  +<helpers/nrf52/SerialBLEInterface.cpp>
+  +<../examples/companion_radio/*.cpp>
+  +<../examples/companion_radio/ui-new/*.cpp>
+lib_deps = ${Promicro.lib_deps}
+  adafruit/RTClib @ ^2.1.3
+  densaugeo/base64 @ ~1.4.0