Add two Breakout examples and the tutorials that build them

Two complete games in `examples/breakout/`, both 100% BASIC: `characters/`
draws its wall in the text grid with two `DATA` sprites for the ball and
paddle, and `sprites/` loads CC0 artwork and captures its whole screen with
`SSHAPE`/`SPRSAV`. They take opposite shapes for reasons that are entirely
this interpreter's, which is what the chapters are for.

`docs/17-tutorial-breakout.md` and `docs/18-tutorial-breakout-artwork.md`
build each one a step at a time, and end in a checklist of the rules a real
program runs into: create every name before the loop starts, write a text row
whole, loop with `GOTO` rather than `DO`, put the float on the left. Every
trap is a runnable block with its own output rather than a claim -- the value
pool dying at four thousand names, the skipped `BEGIN` block that breaks its
caller's `RETURN`, `SSHAPE` ignoring a subscript, `READ`'s single cursor.

Five figures, generated from the listings beside them by `docs_screenshots`,
and a `breakout_art` setup so the ones that load artwork load the example's
own. The character game's wall cannot be photographed -- the screenshot host
omits the text layer on purpose -- so it is shown as compared output instead.

`docs/07-sound.md` never said `SOUND`'s frequency is a SID register value
rather than hertz, which both games depend on. It says so now, with the
conversion from `src/audio_tables.c:84`.

`TODO.md` gains the thirteen defects the two games turned up -- §6 items 30
to 33 and all of §9 -- each with a reduction that fits on a screen, the file
and line of the cause, and what a fix would touch.

Verified: `docs_examples` passes in both build configurations,
`docs_screenshots --check` re-renders all thirteen figures and byte-compares
them, the full 109-test suite passes in both builds, every quoted fragment
was checked to appear verbatim in the listing it came from, and every
relative link and anchor resolves.

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

examples/breakout/sprites/README.md

@@ -0,0 +1,86 @@
+# Breakout, with artwork
+
+Breakout, written entirely in BASIC for the AKGL BASIC interpreter, using downloaded CC0
+sprite artwork for the paddles, the balls and the powerup gems. The bricks, the HUD and
+the lettering are drawn, captured with `SSHAPE` and installed as sprites — which is the
+only way a program in this interpreter can put a picture on the screen.
+
+```sh
+../../../build-akgl/basic breakout.bas
+```
+
+It needs the SDL build. Without a graphics device the first `SPRSAV` refuses by name and
+the program stops there, which is the right thing for it to do.
+
+| Key | Does |
+|---|---|
+| left / right | move the paddle |
+| space | start a game, launch the ball, release a stuck ball |
+| P | pause |
+| S | sound on and off |
+| Q or escape | quit |
+
+Three lives, six rows of bricks worth 60 down to 10 points a row, and a field that cycles
+six rows, then five, then four while the ball gets faster every level up to a ceiling. A
+gem is worth 50 on top of whatever it does.
+
+## The powerups
+
+A broken brick drops a gem about one time in seven, and only ever one at a time. Catch it
+with the paddle. The colour of the gem is what tells you which it is; the banner that
+flashes over the field names it as well.
+
+| Gem | Name | Does | For |
+|---|---|---|---|
+| red | EXPAND | doubles the paddle's width | 20 seconds |
+| yellow | MULTI | throws two more balls off the one in play | until they are lost |
+| green | SLOW | drops the ball's speed to about two thirds | 16 seconds |
+| blue | STICKY | the ball sticks where it lands; space fires it | 18 seconds |
+| purple | CATCH | a second bar appears higher up the field | 24 seconds |
+
+The second bar **mirrors** the paddle rather than following it, so it covers the side you
+just left. A second bar directly above the first is worth nothing; a mirrored one turns a
+dive across the field into a save at both ends.
+
+## How it works, and how to write your own
+
+**[Chapter 18 of the guide](../../../docs/18-tutorial-breakout-artwork.md)** builds this
+program a step at a time: budgeting the eight sprite slots, capturing a drawing into one,
+finding the host's frame boundary with `TI#`, moving everything that is not drawing out of
+the deadline, the stroke font, and the five things in this dialect that do not do what
+they look like.
+
+[Chapter 17](../../../docs/17-tutorial-breakout.md) does the same for
+[`../characters`](../characters), which builds the same game out of the text grid and two
+`DATA` sprites — a completely different program, and the shorter one to read first.
+
+## Where the artwork came from
+
+Kenney's [Puzzle Pack 1](https://kenney.nl/assets/puzzle-pack-1), released under
+[CC0](http://creativecommons.org/publicdomain/zero/1.0/). The files in `art/` are the
+pack's own `PNG/Default/` versions, unmodified, with the pack's licence file beside them.
+`art/PROVENANCE.md` says which file is used for what and why the double-size set is not.
+
+Crediting Kenney is not required by CC0. It is here because it should be.
+
+## What is not in it
+
+Named honestly rather than left to be discovered:
+
+- **No music under the game**, only event sounds and two four-note stings. There is room in
+  the third voice for it; there is not much room anywhere else. 121 of the interpreter's
+  128 variables are spoken for, four more are the ones it makes itself, and the label table
+  is at 61 of 64.
+- **One gem at a time.** There is one sprite slot for it. A brick broken while a gem is
+  falling drops nothing.
+- **Sticky and multiball share one offset.** Two balls stuck to the paddle at once sit on
+  top of each other. It is rare enough that fixing it would cost a variable I do not have.
+- **The score lags a frame behind the bricks.** Only one capture happens per frame and the
+  field goes first, so a brick disappears one frame before the score that counts it. At
+  thirty frames a second nobody can see it, and it is a deliberate trade rather than an
+  oversight.
+- **No high score on disk.** There is no disk.
+
+The eight interpreter defects this game turned up are filed in `TODO.md` §9, each with a
+reduction that fits on a screen, the file and line of the cause, and what a fix would
+touch.