fix(release): publish ESP32 solo binaries, add screenshot to Heltec envs

The solo release job only attached *.uf2 and *.zip, which covered every solo
board back when they were all nRF52. The Heltec V3/V4 .bin images were built
and then silently dropped, so tagging a release produced nothing downloadable
for those two boards.

Releases now carry a single -merged.bin per ESP32 board (bootloader +
partition table + app at 0x0). build.sh also writes an app-only .bin, but
that one needs offset 0x10000 and a bootloader already on the chip, so it's
deleted before upload rather than shipped next to the merged image — flashing
it at 0x0 looks exactly like a bricked device.

Also adds -D ENABLE_SCREENSHOT to both Heltec solo envs. Every other solo env
already had it, so Solo Tools couldn't capture these two boards' displays and
the README's "no special build flags required" was untrue for them. Both envs
rebuilt clean (V3: RAM 58.8%, Flash 43.2%).

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

variants/heltec_v3/platformio.ini

@@ -232,6 +232,7 @@ build_flags =
   -D FIRMWARE_SOLO_BUILD=1
   -D UI_SENSORS_PAGE=1
   -D OLED_MISC_FIXED_FONT=1   ; full Latin/Greek/Cyrillic 6x9 font, ~14 KB flash
+  -D ENABLE_SCREENSHOT        ; same as the other solo builds — Solo Tools reads the framebuffer over USB
   ; CardKB on the second I2C bus. Harmless with nothing plugged in.
   -D ENV_PIN_SDA=3
   -D ENV_PIN_SCL=4