main.c was 2898 lines mixing peripheral init, ISR dispatchers, sensor
handling, state machine, PID control, display, buttons/encoder,
USB-PD negotiation, and persistent settings. This commit extracts
each subsystem into its own .c/.h pair while preserving the
byte-equivalent runtime behavior of the original.

New modules (Core/Src + Core/Inc):
  sensors        thermocouple, bus voltage, MCU temp, heater current
                 ADC sampling and moving-average filters
  heater         TIM1 PWM, ADC2 current measurement, duty cycle math
  handle         JBC handle detection (T210/T245/NT115), cartridge
                 presence, per-handle power limits and PID gains
  state_machine  RUN/STANDBY/SLEEP/HALTED transitions, emergency
                 shutdown checks, delta-temperature fault detection
  encoder        TIM2 quadrature decoding, setpoint write
  buttons        EXTI press handlers, TIM16 debounce, long/short press
  stand          stand-sense GPIO debounce and state transitions
  settings       flash storage of Flash_values struct
  telemetry      rate-limited UART debug packet transmission
  power_source   STUSB4500 USB-PD negotiation
  display_app    main screen render, popups, graph view, uGUI glue
  controller     PID instance, setpoint update, PID_TUNING hooks
  util           clamp()
  version        firmware and hardware version queries

Modified modules:
  buzzer         extended with beep_at_set_temp and ISR entry points

main.c now contains only the CubeMX-generated peripheral inits, the
USER CODE BEGIN 2 / BEGIN WHILE init sequences (byte-exact firing
order with the original), and thin one-line ISR dispatchers that
route HAL callbacks to the owning modules.

Dead code removed in this pass (verified unused in upstream):
  UART_buffer, timer_cleaned, time_to_standby_ms, time_to_sleep_ms,
  and the min()/min3() helpers (only call site was inlined).

No behavior changes, no new features. Code organization only.
RAM and FLASH footprint match upstream within rounding.

Refactoring: modularize main.c into per-subsystem source files