build: phase 2 wave A — services build as packages

init, fat, display, display-demo, device-manager, input, logger, and
the two discovery fillers (acpi, fdt — each exporting an artifact named
"discovery"; the root -Ddiscovery picks which ships) convert to binary
packages on the pci-bus template. init's serial heartbeat flag rides
the dependency options (the root forwards its -Dserial). fat's and
display's unit tests move into their packages and the root aggregate
delegates to them. Boot-image file list unchanged.

system/drivers/virtio-gpu/build.zig

@@ -0,0 +1,37 @@
+//! The virtio-gpu driver as a binary package (docs/build-packages-plan.md,
+//! phase 2): this file names only what is specific to this binary — the shared
+//! recipe and the default-import wiring live in build-support. The root build
+//! consumes the artifact(s) for the boot image.
+
+const std = @import("std");
+const build_support = @import("build-support");
+
+pub fn build(b: *std.Build) void {
+    const protocol = b.dependency("protocol", .{});
+    const device = b.dependency("device", .{});
+
+    const exe = build_support.userBinary(b, .{
+        .name = "virtio-gpu",
+        .root_source_file = b.path("virtio-gpu.zig"),
+        .domains = build_support.domains(b),
+    });
+    build_support.programModule(exe).addImport("pci", device.module("pci"));
+    build_support.programModule(exe).addImport("display-protocol", protocol.module("display-protocol"));
+    build_support.programModule(exe).addImport("scanout-protocol", protocol.module("scanout-protocol"));
+    b.installArtifact(exe);
+
+    // Standalone `zig build test`; the root aggregate depends on this step.
+    const test_step = b.step("test", "Run the virtio-gpu unit tests");
+    for ([_][]const u8{
+        "virtio-gpu-protocol.zig", // virtio-gpu command struct sizes
+        "virtio-pci.zig", // virtio 1.0 PCI transport struct sizes
+    }) |test_root| {
+        const unit_tests = b.addTest(.{
+            .root_module = b.createModule(.{
+                .root_source_file = b.path(test_root),
+                .target = b.resolveTargetQuery(.{}),
+            }),
+        });
+        test_step.dependOn(&b.addRunArtifact(unit_tests).step);
+    }
+}