fix(kernel): Fix PMM memory page table reservation, ATA driver timeout loops, and PCI probe order

- Fix PMM memory map parsing: always reserve lower 8MB on x86_64 to protect bootloader, GDT, IDT, and PML4 page tables
- Fix ATA driver: add bounded timeouts to ata_wait_ready() and ata_wait_drq() to eliminate infinite busy loops
- Fix PCI bus initialization order: probe PCI devices before AHCI/NVMe/NIC storage and network drivers
- Implement missing pmm_get_total_pages() and pmm_get_usable_memory() getters

kernel/main.c

@@ -76,6 +76,10 @@
 #include <userland/shell.h>
 
 static void subos_modular_core_boot(void) {
+#if defined(__x86_64__)
+    pci_init();
+#endif
+
     // Storage & Block Layer
     printk(KERN_INFO "[4/14] Initializing Block Layer & Storage Controllers... ");
     block_init();
@@ -91,7 +95,6 @@ static void subos_modular_core_boot(void) {
     // Bus & Devices
     printk(KERN_INFO "[5/14] Enumerating System Devices, Canvas & PTY... ");
 #if defined(__x86_64__)
-    pci_init();
     virt_init();
     fb_init();
     bochs_vbe_init();