feat(esp32s3): enable and optimize RMT step engine

[tt33415366]

Enable RMT-based stepping engine for ESP32-S3 with simplified implementation and performance optimizations for smoother stepper motion.

Key changes:

• Relocate rmt_engine.c to shared esp32/ directory for multi-platform support
• Enable RMT for ESP32-S3 (MAX_N_RMT = 4)
• Simplified RMT approach: pre-filled SRAM items + direct register trigger
• Use 4MHz RMT clock (clk_div=20) for 0.25us timing precision
• Increase mem_block_num to 3 (192 items) to reduce FIFO reload interrupts
• Increase ACCELERATION_TICKS_PER_SECOND from 100 to 200

Performance improvements:

• Step pulse jitter reduced by ~87% (max glitch: 1535Hz → 203Hz)
• RMS error reduced by 21%
• Quantization error reduced from ±0.5us to ±0.125us

Tested on ESP32-S3 4-axis CNC at 4000Hz step rate.