Add xkeyboard-config: X11 keyboard layouts compiled to Zig

Turn a keycode + modifiers into a character. The input module delivers HID
usage keycodes but nothing mapped them to characters; rather than hand-maintain
layout tables, vendor the X11 xkeyboard-config database and compile it to native
Zig at build time (no X11 runtime), the way make-initial-ramdisk.py packs the
ramdisk.

- tools/make-xkeyboard-config.py: `fetch` downloads the pinned xkeyboard-config
  release (2.44, sha256-verified), resolves the include graph for the configured
  layouts, and vendors only the reached symbols files + keysymdef.h + COPYING +
  PROVENANCE into library/xkeyboard-config/vendor/. `generate` parses that
  (keycodes via a HID->xkb-name table, symbols with include/augment/override and
  per-key type, keysymdef for keysym->Unicode) and emits generated/layouts.zig
  deterministically.
- library/xkeyboard-config/xkeyboard-config.zig: the API over the generated data
  — map(layout, hid_usage, mods) -> { keysym, character }, byName, and the
  level-selection semantics (the generated tables stay pure data). Host tests
  assert US letters/digits with Shift/Caps, GB £ vs US # on Shift+3, and French
  AZERTY q-where-US-has-a — the end-to-end proof of the parse->emit->lookup path.
- build.zig: `xkeyboard-config` + `layouts` modules, the test wired into
  `zig build test`, and a `zig build gen-xkeyboard-config` convenience step.
- Layouts: us, gb, de, fr, es, dvorak. Scope (documented): group 1, no dead-key
  composition, curated key types. Standalone library; wiring it into the input
  path to fill KeyEvent.character is a documented follow-up.

zig build test green (incl. the new keymap tests); regeneration is byte-identical;
full QEMU suite 48/48 (unaffected — no kernel/runtime/service change).

build.zig

@@ -211,6 +211,21 @@ pub fn build(b: *std.Build) void {
         .root_source_file = b.path("library/mmio/mmio.zig"),
     });
 
+    // 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.
+    // No target set, so each inherits its importer's. See library/xkeyboard-config/.
+    const xkb_layouts_module = b.addModule("layouts", .{
+        .root_source_file = b.path("library/xkeyboard-config/generated/layouts.zig"),
+    });
+    const xkeyboard_config_module = b.addModule("xkeyboard-config", .{
+        .root_source_file = b.path("library/xkeyboard-config/xkeyboard-config.zig"),
+        .imports = &.{
+            .{ .name = "layouts", .module = xkb_layouts_module },
+        },
+    });
+    _ = xkeyboard_config_module;
+
     // The POSIX / C compatibility layer, a separate library layered strictly over the
     // runtime (it calls the runtime's IPC/heap, never system calls directly). This is
     // the one place POSIX/C spellings are allowed verbatim — see docs/coding-standards.md
@@ -502,4 +517,25 @@ pub fn build(b: *std.Build) void {
         });
         test_step.dependOn(&b.addRunArtifact(mod_tests).step);
     }
+
+    // The xkeyboard-config keymap tests need its generated `layouts` import wired, so they
+    // don't fit the plain loop above. Its keycode->character assertions are the end-to-end
+    // proof that the xkb-data -> generator -> Zig-lookup pipeline is correct.
+    const xkb_tests = b.addTest(.{
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("library/xkeyboard-config/xkeyboard-config.zig"),
+            .target = target,
+            .optimize = optimize,
+            .imports = &.{
+                .{ .name = "layouts", .module = xkb_layouts_module },
+            },
+        }),
+    });
+    test_step.dependOn(&b.addRunArtifact(xkb_tests).step);
+
+    // Convenience: `zig build gen-xkeyboard-config` regenerates the layout tables from the
+    // vendored data (offline). `fetch` (the network step) stays a manual script run.
+    const gen_xkb = b.addSystemCommand(&.{ "python3", "tools/make-xkeyboard-config.py", "generate" });
+    const gen_xkb_step = b.step("gen-xkeyboard-config", "Regenerate library/xkeyboard-config/generated from the vendored data");
+    gen_xkb_step.dependOn(&gen_xkb.step);
 }