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/src/main.cpp

@@ -153,7 +153,7 @@ void setup() {
 
     s_cv_mutex = xSemaphoreCreateMutex();
 
-    xTaskCreatePinnedToCore(task_camera,   "cam",  4096, nullptr, 2, nullptr, 1);
+    xTaskCreatePinnedToCore(task_camera,   "cam",  8192, nullptr, 2, nullptr, 1);
     xTaskCreatePinnedToCore(task_reporter, "rep",  8192, nullptr, 1, nullptr, 0);
 }