refactor kernel to use device platform discovery

src/device/device.zig

@@ -131,6 +131,14 @@ pub const Device = struct {
         self.resource_count += 1;
         return true;
     }
+
+    /// The device's first resource of `kind`, or null — e.g. a timer's MMIO base.
+    pub fn firstResource(self: *const Device, kind: ResourceKind) ?Resource {
+        for (self.resources[0..self.resource_count]) |r| {
+            if (r.kind == kind) return r;
+        }
+        return null;
+    }
 };
 
 /// Owns the discovered device tree and the allocator its nodes came from.
@@ -146,6 +154,12 @@ pub const DeviceTree = struct {
         return .{ .allocator = allocator, .root = root };
     }
 
+    /// The first device of `class` anywhere in the tree (depth-first), or null —
+    /// how the kernel pulls e.g. the HPET or IOAPIC MMIO base out of discovery.
+    pub fn firstOfClass(self: *const DeviceTree, class: DeviceClass) ?*Device {
+        return firstOfClassIn(self.root, class);
+    }
+
     /// Allocate a device and append it under `parent`, returning it so the caller
     /// can attach resources/ids. Appended at the tail so a dump reads in the order
     /// devices were discovered.
@@ -176,6 +190,15 @@ pub const DeviceTree = struct {
     }
 };
 
+fn firstOfClassIn(node: *Device, class: DeviceClass) ?*Device {
+    var child = node.first_child;
+    while (child) |c| : (child = c.next_sibling) {
+        if (c.class == class) return c;
+        if (firstOfClassIn(c, class)) |found| return found;
+    }
+    return null;
+}
+
 fn dumpNode(dev: *const Device, depth: usize, emit: *const fn ([]const u8) void) void {
     const indent = @min(depth * 2, 40);