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.

test/system/services/thread-test/build.zig

@@ -0,0 +1,16 @@
+//! The thread-test test fixture 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 exe = build_support.userBinary(b, .{
+        .name = "thread-test",
+        .root_source_file = b.path("thread-test.zig"),
+        .domains = build_support.domains(b),
+    });
+    b.installArtifact(exe);
+}