build: single-source the default import set; delegate library tests

Review findings from the pilot: the 17-module default import set was
maintained twice (root's array and build-support's userBinary) — drift
would silently make packaged binaries differ from root-built ones. It
now lives once, as build-support.defaultImports; root and userBinary
both draw from it, and root's user-binary wrapper names the shim +
link script through the kernel package like build-support does.
The root aggregate test step likewise duplicated the library domains'
14 test definitions; it now depends on each domain's own standalone
test step (host-only, so root's -Dtarget/-Doptimize deliberately do
not reach them), and the client domain gains an empty test step for
uniformity. Boot-image file list unchanged.

library/client/build.zig

@@ -26,4 +26,10 @@ pub fn build(b: *std.Build) void {
             .{ .name = "input-protocol", .module = protocol.module("input-protocol") },
         },
     });
+
+    // Standalone `zig build test`, kept for uniformity across the domains (the
+    // root aggregate depends on every domain's test step). The clients have no
+    // host-runnable unit tests yet — they are thin IPC conversation wrappers —
+    // so the step is empty until one grows some.
+    _ = b.step("test", "Run the client unit tests (none yet)");
 }