Implement ISR stubs and PIC driver for hardware interrupt handling (AI)

- Reworked IDT initialization to register all 32 CPU exception handlers (ISR 0-31)
  and 16 hardware interrupt handlers (IRQ 0-15, mapped to IDT entries 32-47).
- Created assembly stubs in interrupts.S using macros for ISRs with and without
  error codes, plus IRQ stubs. All route through a common stub that saves
  registers, loads kernel data segment, and calls the C handler.
- Added isr.c with a unified interrupt dispatcher that handles both exceptions
  (halts on fault) and hardware IRQs (sends EOI via PIC).
- Implemented PIC (8259) driver in pic.c with full initialization sequence that
  remaps IRQ 0-7 to IDT 32-39 and IRQ 8-15 to IDT 40-47. Includes mask/unmask
  and EOI support.
- Extracted port I/O primitives (inb, outb, io_wait) into port_io.h header for
  reuse across drivers.
- Kernel now initializes PIC after IDT and enables interrupts with STI.

src/kernel.c

@@ -3,11 +3,8 @@
 #include <stddef.h>
 #include "gdt.h"
 #include "idt.h"
-
-static inline void outb(uint16_t port, uint8_t val)
-{
-    asm volatile ( "outb %b0, %w1" : : "a"(val), "Nd"(port) );
-}
+#include "pic.h"
+#include "port_io.h"
 
 void offset_print(const char *str)
 {
@@ -46,6 +43,13 @@ void kernel_main(uint32_t magic, uint32_t addr) {
     
     init_idt();
     offset_print("IDT initialized\n");
-    
+
+    init_pic();
+    offset_print("PIC initialized\n");
+
+    /* Enable interrupts */
+    asm volatile("sti");
+    offset_print("Interrupts enabled\n");
+
     offset_print("Hello, world\n");
 }