Decode PCI class codes in device discovery

`pci_device` alone says nothing — an ISA bridge, an AHCI controller, and an xHCI USB
controller are all just `pci_device` by DeviceClass. The identity lives in the 24-bit
class code (base class / subclass / prog-IF) that discovery already recorded in
ids.pci_class but the dump threw away.

New system/devices/pci-class.zig (module `pci-class`): pure reference data from the PCI
spec (per the OSDev PCI table) decoding the triple into names — className, subclassName,
progIfName, plus ClassCode.unpack. No hardware access, so it's shared by kernel
discovery and any future user-space PCI tool. device_tree.dump now prints, under each
PCI function, its class/subclass/prog-IF as both hex and name:

    pci0:00:1f.0 [pci_device]
      class 0x06 (Bridge)  subclass 0x01 (ISA Bridge)  progif 0x00
    pci0:00:1f.2 [pci_device]
      class 0x01 (Mass Storage Controller)  subclass 0x06 (Serial ATA Controller)  progif 0x01 (AHCI 1.0)

Host test covers the decoder (bridge/AHCI/xHCI, the "Other" 0x80 convention, unknowns).
Suite 41/41 plus host tests.

build.zig

@@ -116,6 +116,11 @@ pub fn build(b: *std.Build) void {
     const device_abi_module = b.addModule("device-abi", .{
         .root_source_file = b.path("system/devices/device-abi.zig"),
     });
+    // PCI class-code decoding (class/subclass/prog-IF -> names). Pure reference data,
+    // shared by kernel discovery (the device-tree dump) and any user-space PCI tool.
+    const pci_class_module = b.addModule("pci-class", .{
+        .root_source_file = b.path("system/devices/pci-class.zig"),
+    });
 
     // Kernel tunables (maximum_cpus, stack sizes, tick rate). A dependency-free module of
     // compile-time constants, imported wherever a knob is read; keeps the trade-offs
@@ -153,6 +158,7 @@ pub fn build(b: *std.Build) void {
             .{ .name = "boot-handoff", .module = boot_handoff_module }, // BootInformation (carries the ACPI RSDP), physicalToVirtual
             .{ .name = "abi", .module = abi_module }, // acpi.zig works in page_size units
             .{ .name = "device-abi", .module = device_abi_module }, // device-model's DeviceClass/ResourceKind live here
+            .{ .name = "pci-class", .module = pci_class_module }, // decode PCI class codes in the device dump
             .{ .name = "parameters", .module = parameters_module }, // maximum_cpus (the discovery pool)
         },
     });
@@ -439,6 +445,7 @@ pub fn build(b: *std.Build) void {
         "system/boot-handoff.zig",
         "system/abi.zig",
         "system/devices/device-abi.zig",
+        "system/devices/pci-class.zig", // class/subclass/prog-IF name decoding
         "library/mmio/mmio.zig", // barriers assemble + registers round-trip
     }) |root| {
         const mod_tests = b.addTest(.{