refactor: enforce max 8 code lines, add docstrings, fix warnings across all Rust and C SDK projects

Rust (all 15 projects):
- Refactored overlength functions: format_counter, format_u8, format_f32_1,
  format_u32_minimal, gpio_drive, read_sensor, poll_sensor, format_round_trip,
  format_u32, prepare_write_buf, write_min_digits, write_temp, UartDriver::init,
  init_spi, angle_to_pulse_us, compute_servo_level
- Added 200+ docstrings to test functions, mock structs, impl blocks
- Fixed pub static comments (//) to doc comments (///) in all main.rs files
- Fixed helper function ordering (helpers above callers)
- Fixed Fn(u32) -> FnMut(u32) bound in button poll_button
- Moved OneShot trait import from main.rs to board.rs in adc project
- Added unsafe {} blocks in flash unsafe fn bodies (Rust 2024 edition)
- Removed unused hal::Clock imports from pwm/servo main.rs
- All 15 projects build with zero errors and zero warnings

C Pico SDK (all 15 projects):
- Added docstrings to all public functions, macros, and static variables
- All 15 projects rebuilt with zero errors

Cleanup:
- Removed build/ and target/ directories from git tracking
- Added target/ to .gitignore
- Deleted temporary fix_rust_docs.py script

drivers/0x0d_timer/timer.c

@@ -30,8 +30,11 @@
 #include "timer.h"
 #include "pico/time.h"
 
+/** @brief Pico SDK repeating timer handle */
 static repeating_timer_t g_timer;
+/** @brief Flag indicating whether the timer is running */
 static bool g_timer_active = false;
+/** @brief User-provided timer callback function */
 static timer_driver_callback_t g_user_callback = NULL;
 
 /**
@@ -47,6 +50,17 @@ static bool _timer_shim(repeating_timer_t *rt) {
     return false;
 }
 
+/**
+ * @brief Start a repeating hardware timer that fires the given callback
+ *
+ * Schedules callback to be invoked every period_ms milliseconds using
+ * the Pico SDK add_repeating_timer_ms() API. The timer continues firing
+ * until timer_driver_cancel() is called.
+ *
+ * @param period_ms Interval between callbacks in milliseconds (positive value)
+ * @param callback  Function to call on each timer expiry; must return true to
+ *                  continue repeating, false to stop
+ */
 void timer_driver_start(int32_t period_ms, timer_driver_callback_t callback) {
     if (g_timer_active) {
         cancel_repeating_timer(&g_timer);
@@ -56,6 +70,12 @@ void timer_driver_start(int32_t period_ms, timer_driver_callback_t callback) {
     g_timer_active = add_repeating_timer_ms(period_ms, _timer_shim, NULL, &g_timer);
 }
 
+/**
+ * @brief Cancel the active repeating timer
+ *
+ * Stops the timer started by timer_driver_start(). Safe to call even if the
+ * timer has already fired and self-cancelled.
+ */
 void timer_driver_cancel(void) {
     if (!g_timer_active)
         return;