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>

docs/19-tutorial-sidescroller.md

@@ -23,10 +23,20 @@ cmake --build build --parallel --target sidescroller
 ./build/examples/sidescroller/sidescroller
 ```
 
-Arrow keys to run, space to jump, and holding space longer jumps higher. Three flags exist
-for the smoke test and are useful by hand too: `--assets DIR` points somewhere other than
-the compiled-in asset directory, `--frames N` (or `AKGL_SIDESCROLLER_FRAMES`) exits after
-that many frames, and `--autoplay` drives the player from a script instead of the keyboard.
+### Controls
+
+| Key | Gamepad | Does |
+|---|---|---|
+| Left arrow | D-pad left | Run left |
+| Right arrow | D-pad right | Run right |
+| Space | South button (A) | Jump — hold it longer to jump higher |
+
+Close the window to quit; there is no key bound to it.
+
+Three flags exist for the smoke test and are useful by hand too: `--assets DIR` points
+somewhere other than the compiled-in asset directory, `--frames N` (or
+`AKGL_SIDESCROLLER_FRAMES`) exits after that many frames, and `--autoplay` drives the
+player from a script instead of the keyboard.
 
 ## The level
 
@@ -745,22 +755,37 @@ set_tests_properties(example_sidescroller PROPERTIES
 ```
 
 Four seconds of scripted play under the headless drivers. `--autoplay` holds *right* and
-requests a jump every forty-five frames by calling the handlers a keyboard would have
-called:
+taps jump every forty-five frames, and it does it by pushing synthetic events through
+`akgl_controller_handle_event` rather than by calling the handlers:
 
 ```c excerpt=examples/sidescroller/player.c
     if ( frame == 1 ) {
-	PASS(errctx, akgl_actor_cmhf_right_on(ss_game.player, &synthetic));
-    }
-    if ( (frame % SS_AUTOPLAY_JUMP_PERIOD) == 0 ) {
-	ss_game.jump_requested = true;
+	synthetic.type = SDL_EVENT_KEY_DOWN;
+	synthetic.key.which = SS_AUTOPLAY_KBID;
+	synthetic.key.key = SDLK_RIGHT;
+	PASS(errctx, akgl_controller_handle_event((void *)&akgl_game.state, &synthetic));
+	FAIL_ZERO_RETURN(
+	    errctx,
+	    AKGL_BITMASK_HAS(ss_game.player->state, AKGL_ACTOR_STATE_MOVING_RIGHT),
+	    AKGL_ERR_BEHAVIOR,
+	    "the right arrow did not reach the player; the control map matched nothing"
+	    );
     }
 ```
 
-That works because a control-map handler takes the actor and an event, and the keyboard
-handlers do not read the event beyond requiring one to be there. In four seconds the script
-walks the level, jumps, collides with terrain, collects a coin, falls in the pit and
-respawns — so the smoke test exercises the collision and the pickup, not just the startup.
+**That distinction is the whole value of the smoke test, and the first version got it
+wrong.** Calling the handlers directly is easier and it works — but it skips the binding
+table, which is the part with something to get wrong. This game shipped with a control map
+that matched no keyboard on earth, and a smoke test that called the handlers passed anyway.
+A test that cannot fail is not a test, so the script drives the real dispatch and then
+asserts the press actually arrived. Break the binding and the run exits non-zero naming the
+reason.
+
+`SS_AUTOPLAY_KBID` is deliberately not `0`: the map binds keyboard `0` meaning *any*
+keyboard, and driving it from a non-zero device id is what proves that. In four seconds the
+script walks the level, jumps, collides with terrain, collects a coin, falls in the pit and
+respawns — so the smoke test exercises the input path, the collision and the pickup, not
+just the startup.
 
 A run ends with a line naming what happened, which is worth reading when a change moves the
 feel: