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/isr.c

@@ -0,0 +1,75 @@
+#include "isr.h"
+#include "pic.h"
+#include <stdint.h>
+
+/* Forward declaration for kernel panic or similar */
+void offset_print(const char *str);
+void print_hex(uint32_t val);
+
+/* Exception messages */
+char *exception_messages[] = {
+    "Division By Zero",
+    "Debug",
+    "Non Maskable Interrupt",
+    "Breakpoint",
+    "Into Detected Overflow",
+    "Out of Bounds",
+    "Invalid Opcode",
+    "No Coprocessor",
+    "Double Fault",
+    "Coprocessor Segment Overrun",
+    "Bad TSS",
+    "Segment Not Present",
+    "Stack Fault",
+    "General Protection Fault",
+    "Page Fault",
+    "Unknown Interrupt",
+    "Coprocessor Fault",
+    "Alignment Check",
+    "Machine Check",
+    "Reserved",
+    "Reserved",
+    "Reserved",
+    "Reserved",
+    "Reserved",
+    "Reserved",
+    "Reserved",
+    "Reserved",
+    "Reserved",
+    "Reserved",
+    "Reserved",
+    "Reserved",
+    "Reserved"
+};
+
+void isr_handler(registers_t *regs)
+{
+    // If it's a hardware interrupt (IRQ), we must acknowledge it
+    if (regs->int_no >= 32 && regs->int_no < 48) {
+        // Send EOI to PIC (IRQ number 0-15)
+        pic_send_eoi(regs->int_no - 32);
+    
+        // Here we would call the registered handler for this IRQ
+        // For now, just print something for the timer tick so we know it works, 
+        // but limit it to avoid flooding the log.
+        if (regs->int_no == 32) {
+            // Timer tick - do nothing verbose
+            // offset_print("."); 
+        } else if (regs->int_no == 33) {
+            // Keyboard
+            offset_print("Keyboard IRQ!\n");
+        }
+        return;
+    }
+
+    offset_print("received interrupt: ");
+    print_hex(regs->int_no);
+    offset_print("\n");
+
+    if (regs->int_no < 32)
+    {
+        offset_print(exception_messages[regs->int_no]);
+        offset_print(" Exception. System Halted!\n");
+        for (;;) ;
+    }
+}