build: phase 1 — library domains become packages

Each library domain (kernel, device, client, protocol, csv,
xkeyboard-config) now owns a build.zig + build.zig.zon that wires and
exports its modules, with a standalone `zig build test` per domain.
The kernel package also exports abi (source stays in system/abi.zig) so
every consumer names one module instance. The root build swaps its
createModule calls for b.dependency(...).module(...) — no binary moves;
the root is the pilot consumer (docs/build-packages-plan.md). Path
dependencies deduplicate by resolved location, so the diamond
(root -> device -> kernel, root -> kernel) yields a single instance of
each module. Boot-image file list unchanged.

library/xkeyboard-config/build.zig

@@ -0,0 +1,31 @@
+//! The "xkeyboard-config" library domain: keyboard layouts compiled from the
+//! X11 xkeyboard-config database into native Zig (keycode + modifiers ->
+//! keysym/character). The `layouts` tables are generated by
+//! tools/make-xkeyboard-config.py; `xkeyboard-config` is the hand-written API
+//! over them.
+
+const std = @import("std");
+
+pub fn build(b: *std.Build) void {
+    const layouts = b.addModule("layouts", .{
+        .root_source_file = b.path("generated/layouts.zig"),
+    });
+    _ = b.addModule("xkeyboard-config", .{
+        .root_source_file = b.path("xkeyboard-config.zig"),
+        .imports = &.{.{ .name = "layouts", .module = layouts }},
+    });
+
+    // Standalone `zig build test` for this domain alone; the root build keeps
+    // its aggregate test step. The keycode->character assertions are the
+    // end-to-end proof that the xkb-data -> generator -> Zig-lookup pipeline
+    // is correct.
+    const test_step = b.step("test", "Run the xkeyboard-config unit tests");
+    const xkb_tests = b.addTest(.{
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("xkeyboard-config.zig"),
+            .target = b.resolveTargetQuery(.{}),
+            .imports = &.{.{ .name = "layouts", .module = layouts }},
+        }),
+    });
+    test_step.dependOn(&b.addRunArtifact(xkb_tests).step);
+}

library/xkeyboard-config/build.zig.zon

@@ -0,0 +1,8 @@
+.{
+    .name = .xkeyboard_config,
+    .version = "0.0.0",
+    .fingerprint = 0xea5abe82f08b6eae, // Changing this has security and trust implications.
+    .minimum_zig_version = "0.16.0",
+    .dependencies = .{},
+    .paths = .{""},
+}