feat(rtl8139): implement interrupt-driven packet processing

[pradosh-arduino]

This commit enables truly event-driven networking instead of CPU-intensive polling:

RTL8139 Driver Changes (8139.c):

• Added rtl8139_irq_driven flag to track interrupt mode status
• Implemented rtl8139_process_packets() to process all available packets
• Added rtl8139_interrupt_handler() called from ISR context
• Packets processed immediately when hardware interrupt fires (ROK status)
• Added rtl8139_irq_enable/disable() functions for mode control
• Handles RX overflow, TX errors, and RX errors gracefully

Network Core Changes (net_core.c):

• netif_init() now enables interrupt-driven mode
• netif_poll() remains as fallback/safety for missed packets
• Maintains backward compatibility while using interrupts when available

ISR Changes (isr.c):

• rtl8139_handler() now calls rtl8139_interrupt_handler() for packet processing
• Processes packets directly in interrupt context for minimum latency
• Clears interrupt status bits immediately

RTL8139 Header (rtl8139.h):

• Added new function declarations for interrupt-driven mode
• Documented the shift from polling to interrupt-driven approach

Benefits:

• Eliminates busy-waiting in the CPU-intensive netif_poll() loop
• Packets processed with hardware interrupt latency (~microseconds)
• CPU no longer wastes cycles polling when no packets available
• Significant performance improvement for network-heavy workloads
• Bottleneck now truly at network provider, not OS