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).

docs/input.md

@@ -118,6 +118,12 @@ the service delivers to its endpoint, which only the same thread could receive).
   fan-out and per-device routing are real; only the bytes are placeholder. **Follow-up:** the
   bus binds IRQ1/IRQ12, reads port 0x60, and `ps2-library` decodes scan-set-1 →
   keycodes and mouse packets; the drivers publish decoded events.
+- **Keycode → character** is a keymap, and danos has one:
+  [`library/xkeyboard-config`](../library/xkeyboard-config/README.md) compiles the X11
+  xkeyboard-config layouts (us, gb, de, fr, …) to native Zig —
+  `xkb.map(layout, keycode, mods)` → keysym + Unicode character. Wiring it in to fill a
+  `key_press` event's `character` (in the keyboard driver, or a small keymap service) is the
+  natural next step.
 - **Drop-oldest under overflow** is a defined loss; the 16-slot ring absorbs normal bursts.
   Real backpressure/flow-control is future work.
 - **`publish` is unauthenticated** — any process may publish, consistent with the current