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.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 = .{""},
+}