Let a control map listen to any keyboard, and fix the dead arrow keys

The sidescroller's controls did nothing. `akgl_controller_handle_event` matched
`event->key.which == curmap->kbid` exactly, with no way to say "whatever
keyboard the player is typing on", so the game did the obvious thing and bound
`SDL_GetKeyboards()[0]`.

That cannot work. The id a key event *carries* is chosen by the video backend
and is not required to be an id `SDL_GetKeyboards()` reports. On X11 without
XInput2 every key event carries `SDL_GLOBAL_KEYBOARD_ID`, which is 0, while
`SDL_AddKeyboard` registers the attached keyboard as `SDL_DEFAULT_KEYBOARD_ID`,
which is 1; with XInput2 the events carry the physical slave device's
`sourceid` while `SDL_GetKeyboards()[0]` may be the master. Either way the map
matched nothing and every key press was dropped.

A `kbid` or `jsid` of 0 now matches any device of that kind. 0 is safe to spend:
SDL documents `which` as 0 when the source is unknown or virtual, and joystick
ids start at 1, so no real device is 0. A non-zero id still matches only that
device, which is what keeps two local players on two keyboards apart, and there
is a test for that half too. `util/charviewer.c` already passed 0 and depended
on the old behaviour by accident; it works under XInput2 now as well.

Two reasons this shipped, both closed:

- Every test in tests/controller.c dispatched an event whose id equalled the id
  the map was bound with, so none of them could see it.
- The sidescroller's smoke test called the control handlers directly instead of
  dispatching events, so it passed against a control map that matched nothing.
  It now pushes synthetic events through akgl_controller_handle_event from a
  deliberately non-zero device id and fails if the press does not arrive.
  Verified by breaking the binding and watching the run exit non-zero.

`test_controller_wildcard_device_ids` was written first and failed against the
unfixed library with "a map bound to keyboard 0 ignored a key press from device
11", which is the whole reason to trust it now that it passes.

The controls were documented, in one sentence. Chapter 19 has a proper table of
them, and chapter 15 explains why not to reach for SDL_GetKeyboards(). The
header said the match was exact and now says what it does.

Co-Authored-By: Claude Code <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

include/akgl/controller.h

@@ -87,7 +87,7 @@ typedef struct {
     akgl_Actor *target;                         /**< The actor these bindings drive. A `NULL` target makes the whole map inert, which is how an unused slot is spelled. */
     uint16_t nextMap;                           /**< Number of bindings in use; the index akgl_controller_pushmap writes to next. */
     akgl_Control controls[AKGL_MAX_CONTROLS];   /**< The bindings, scanned in order. */
-    SDL_KeyboardID kbid;                        /**< Keyboard this map listens to. A keyboard event from any other id is ignored, which is what keeps two players on two keyboards apart. */
+    SDL_KeyboardID kbid;                        /**< Keyboard this map listens to, or 0 for any. A non-zero id ignores every other keyboard, which is what keeps two players on two keyboards apart. */
     SDL_JoystickID jsid;                        /**< Gamepad this map listens to, matched the same way. */
     SDL_MouseID mouseid;                        /**< Mouse this map listens to. Declared but not yet consulted. */
     SDL_PenID penid;                            /**< Pen this map listens to. Declared but not yet consulted. */
@@ -249,10 +249,14 @@ akerr_ErrorContext AKERR_NOIGNORE *akgl_controller_pushmap(int controlmapid, akg
  * @param actorname    Registry name of the actor to drive. Required in practice,
  *                     though a `NULL` is reported as "not found" rather than as
  *                     a null pointer.
- * @param kbid         SDL keyboard id to listen to. Only events from this
- *                     keyboard match; see akgl_controller_list_keyboards for
- *                     how to find it.
- * @param jsid         SDL gamepad id to listen to, matched the same way.
+ * @param kbid         SDL keyboard id to listen to, or **0 for any keyboard**,
+ *                     which is what a single-player game wants. Do not pass
+ *                     `SDL_GetKeyboards()[0]`: that reports what is attached,
+ *                     while the id an event carries is chosen by the video
+ *                     backend, and the two need not agree. See
+ *                     akgl_controller_list_keyboards.
+ * @param jsid         SDL gamepad id to listen to, or 0 for any gamepad,
+ *                     matched the same way.
  * @return `NULL` on success, otherwise an error context owned by the caller.
  * @throws AKERR_OUTOFBOUNDS If @p controlmapid is at or above
  *         #AKGL_MAX_CONTROL_MAPS, or if the map cannot hold eight more bindings.