Add two tutorial games that build, run in CI, and cannot drift

examples/sidescroller and examples/jrpg are complete programs, built with the
library and exercised headless by ctest. The chapters quote them with
`c excerpt=examples/...` blocks rather than restating the code, so a chapter
cannot drift from a program that compiles -- the excerpt check fails the moment
the source moves. 34 excerpts in one chapter, 21 in the other.

The two are complementary. The sidescroller is the physics tutorial: gravity, a
jump, coins, hazards. The JRPG is the content-pipeline tutorial: a town map,
NPCs spawned from map objects, four-way per-facing animation, a text box, a
follower.

Both smoke tests drive real SDL_Events through akgl_controller_handle_event and
step the physics clock at a fixed 1/60s rather than sleeping, so a scripted run
is deterministic and finishes in under five seconds.

Writing them is what turned up most of the defects recorded in the next commit,
because a game exercises paths a unit test does not. Each workaround says in the
chapter which library gap forced it:

- collision is written in a custom movementlogicfunc, because
  akgl_physics_arcade_collide raises AKERR_API and akgl_physics_simulate never
  calls collide at all;
- the sidescroller cancels the step's own gravity when it blocks downward,
  because otherwise a quarter-pixel of penetration makes the *horizontal* sweep
  report blocked and the character walks backwards a tile at a time;
- both clear movement_controls_face on every map-spawned actor, because the
  default facefunc leaves a stopped actor with no facing bit, no sprite, and no
  draw;
- the JRPG's follower gets a renderfunc that nulls obj->parent for the duration
  of the draw, because a child's offset is counted twice.

Assets are CC0 from three Kenney packs, vendored with per-pack licence text,
per-file provenance, and the geometry contract in
docs/tutorials/assets/README.md. CC0 specifically rather than merely free: a
reader who copies a tutorial into their own game inherits no obligation.
scripts/fetch_tutorial_assets.sh refreshes them in the shape
mkcontrollermappings.sh was fixed into for 0.5.0 -- it checks curl's status,
refuses a pack page that does not say CC0, verifies the archive and the staged
dimensions, and leaves the tracked bytes untouched on any failure. Both failure
paths were tested, and a no-op refresh is byte-identical.

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

examples/sidescroller/CMakeLists.txt

@@ -0,0 +1,48 @@
+# The sidescroller tutorial game.
+#
+# A real target, built by default, so docs/19-tutorial-sidescroller.md cannot
+# quote a program that does not compile -- and registered as a headless smoke
+# test, so it cannot quote one that does not run.
+
+get_filename_component(SS_ASSET_DIR
+  "${CMAKE_CURRENT_SOURCE_DIR}/../../docs/tutorials/assets/sidescroller" ABSOLUTE)
+
+add_executable(sidescroller
+  main.c
+  collision.c
+  player.c
+  actors.c
+)
+
+target_include_directories(sidescroller PRIVATE "${CMAKE_CURRENT_SOURCE_DIR}")
+target_compile_options(sidescroller PRIVATE ${AKGL_WARNING_FLAGS})
+
+# Baked in rather than looked up at runtime, so the smoke test can be launched
+# from any working directory. `--assets DIR` overrides it for a reader who has
+# moved the art somewhere else.
+target_compile_definitions(sidescroller PRIVATE "SS_ASSET_DIR=\"${SS_ASSET_DIR}\"")
+
+target_link_libraries(sidescroller
+  PRIVATE akstdlib::akstdlib akerror::akerror akgl SDL3::SDL3 SDL3_ttf::SDL3_ttf
+          SDL3_image::SDL3_image SDL3_mixer::SDL3_mixer jansson::jansson -lm)
+
+# A vendored build leaves the SDL satellite libraries in per-project
+# subdirectories that are not on the loader's default search path, which is the
+# same problem AKGL_VENDORED_RPATH solves for the test executables.
+if(AKGL_VENDORED_DEPENDENCIES)
+  set_target_properties(sidescroller PROPERTIES BUILD_RPATH "${AKGL_VENDORED_RPATH}")
+endif()
+
+# Four seconds of scripted play under the headless drivers: the level loads, the
+# player walks and jumps, the swept collision runs against real terrain, and the
+# program tears down and exits 0. A tutorial that stops working fails here rather
+# than in front of a reader.
+#
+# add_test() rather than _add_test(): the root CMakeLists.txt shadows it only
+# while this project is top-level, and by the time examples/ is added the
+# suppression has already been lifted. An embedded build gets the builtin.
+add_test(NAME example_sidescroller COMMAND sidescroller --frames 240 --autoplay)
+set_tests_properties(example_sidescroller PROPERTIES
+  TIMEOUT 120
+  ENVIRONMENT "SDL_VIDEODRIVER=dummy;SDL_RENDER_DRIVER=software;SDL_AUDIODRIVER=dummy"
+)