fix(gamepad): build libevdev from a vendored copy

The Linux gamepad backend included <libevdev/libevdev.h> from
/usr/include/libevdev-1.0 and linked the system libevdev, but the
Flatpak SDK crafter-build runs in (org.freedesktop.Sdk 26.08) ships no
libevdev headers, so Crafter.Graphics stopped building there.
lib/libevdev now carries libevdev 1.14.0 (MIT), built once as its own
static library that Crafter.Graphics depends on. As part of cfg it would
be compiled again in every test configuration, since they copy
cfg.cFiles.

libevdev's generated event-names.h names every input code by its macro,
so the library compiles against the kernel input headers that table was
generated from (lib/libevdev/kernel-headers), not the target's. That
directory is deliberately not called include: Crafter.Build puts every
include directory of a dependency on its consumers' include path, where
those headers would shadow each consumer's own <linux/input.h>. The
paths are absolute because Crafter.Build only resolves the returned
configuration's paths against the project root.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/libevdev/README.md

@@ -0,0 +1,30 @@
+# libevdev (vendored)
+
+The Linux gamepad backend (`implementations/Crafter.Graphics-Gamepad.cpp`) parses evdev events with
+[libevdev](https://gitlab.freedesktop.org/libevdev/libevdev). It is compiled into Crafter.Graphics from
+this directory instead of being linked from the system. The Flatpak SDK that crafter-build runs in has no
+libevdev headers, and a vendored copy builds the same way for every target, including cross sysroots.
+
+- Upstream: `https://gitlab.freedesktop.org/libevdev/libevdev.git`, tag `libevdev-1.14.0`
+  (commit `d9af2e4580297e9c14715fef06621dd39d9d8a3a`). MIT, see `COPYING`.
+- `libevdev/`: the core library sources, unchanged. The uinput half isn't used and isn't included.
+- `libevdev/event-names.h`: generated, the way libevdev's meson build does it, from the kernel headers
+  below.
+- `libevdev/config.h`: what libevdev's meson build writes on Linux (`_GNU_SOURCE`).
+- `kernel-headers/linux/`: the kernel uapi `input.h` and `input-event-codes.h` that ship with that
+  libevdev release. libevdev compiles against its own copy (its meson build puts them first on the path),
+  because `event-names.h` names every code by its macro and has to see the same set of codes. Only the
+  libevdev library sees them, and the directory isn't called `include`, because Crafter.Build puts every
+  `include` directory of a dependency on its consumers' include path.
+
+`project.cpp` builds these sources once, as their own static library, which Crafter.Graphics depends on.
+
+## Updating
+
+Copy `COPYING`, the files under `libevdev/` and upstream's `include/linux/linux/{input.h,input-event-codes.h}`
+(into `kernel-headers/linux/`) from the new release, keep `config.h`, then regenerate the name table:
+
+```sh
+python3 libevdev/make-event-names.py kernel-headers/linux/input.h kernel-headers/linux/input-event-codes.h \
+    > libevdev/event-names.h
+```