fix: three stack overflows crashing firmware on TimerCamera-F

loopTask: cv_init() created a CVState{} temporary (9KB background
array) on the stack — fixed by initializing members directly.

cam task: cv_process() had uint8_t fg[CV_PIXELS] (9KB) as a local
variable — made static, matching the existing fg_copy fix.

cam task stack bumped from 4096 to 8192 for headroom.

Also: switch to 4MB OTA partition table (TimerCamera-F has 4MB flash,
not 8MB), add CONFIG_ARDUINO_LOOP_STACK_SIZE=16384 build flag,
upload_speed=115200 and --no-stub for reliable CH340 flashing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

firmware/platformio.ini

@@ -6,15 +6,17 @@ default_envs = timercam
 platform = espressif32@6.6.0
 board = m5stack-timer-cam
 framework = arduino
-board_build.partitions = partitions_8mb_ota.csv
+board_build.partitions = partitions_4mb_ota.csv
 build_flags =
     -DBOARD_HAS_PSRAM
     -mfix-esp32-psram-cache-issue
     -DCORE_DEBUG_LEVEL=3
     -DCONFIG_BT_NIMBLE_ENABLED=1
     -DCONFIG_SPIRAM_USE_MALLOC=1
+    -DCONFIG_ARDUINO_LOOP_STACK_SIZE=16384
 monitor_speed = 115200
-upload_speed = 921600
+upload_speed = 115200
+upload_flags = --no-stub
 lib_deps =
     tzapu/WiFiManager@^2.0.17
     bblanchon/ArduinoJson@^7.0.0