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/art/License.txt

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+	
+
+	Puzzle Pack (1.1)
+
+	Created/distributed by Kenney (www.kenney.nl)
+
+			------------------------------
+
+	License: (Creative Commons Zero, CC0)
+	http://creativecommons.org/publicdomain/zero/1.0/
+
+	This content is free to use in personal, educational and commercial projects.
+	Support us by crediting Kenney or www.kenney.nl (this is not mandatory)
+
+			------------------------------
+
+	Donate:   http://support.kenney.nl
+	Patreon:  http://patreon.com/kenney/
+
+	Follow on Twitter for updates:
+	http://twitter.com/KenneyNL

examples/breakout/sprites/art/PROVENANCE.md

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+# Where this art came from
+
+Every PNG in this directory is from **Kenney's Puzzle Pack 1**, released into the public
+domain under [Creative Commons Zero](http://creativecommons.org/publicdomain/zero/1.0/).
+`License.txt` is the pack's own licence file, copied here unedited.
+
+* Source: <https://kenney.nl/assets/puzzle-pack-1>
+* Downloaded: 2026-08-01, `kenney_puzzle-pack-1.zip`
+* Author: Kenney (<https://www.kenney.nl>)
+* Licence: CC0 1.0. Crediting is not required; it is here because it should be.
+
+The files are the pack's `PNG/Default/` versions, byte for byte — nothing is resized,
+recoloured or re-encoded, so the checksum of any of them still matches the distributed
+archive.
+
+| File | Size | Used for |
+|---|---|---|
+| `paddleBlu.png`                     | 104x24 | the player's paddle |
+| `paddleRed.png`                     | 104x24 | the second paddle, from the CATCHER powerup |
+| `ballBlue.png`                      | 22x22  | the ball |
+| `ballGrey.png`                      | 22x22  | the extra balls from MULTIBALL |
+| `element_red_polygon_glossy.png`    | 48x46  | the EXPAND powerup |
+| `element_yellow_polygon_glossy.png` | 48x46  | the MULTIBALL powerup |
+| `element_green_polygon_glossy.png`  | 48x46  | the SLOW powerup |
+| `element_blue_polygon_glossy.png`   | 48x46  | the STICKY powerup |
+| `element_purple_polygon_glossy.png` | 48x46  | the CATCHER powerup |
+
+The pack's `PNG/Double/` versions are twice these sizes and are the wrong scale for an
+800x600 field. `SPRITE`'s x- and y-expand bits would double them again, which is the only
+scaling this interpreter has — there is no way to make a sprite *smaller* — so the
+`Default` set is the one that fits.
+
+Everything else on screen — the bricks, the walls, the HUD lettering, the title — is drawn
+by the program with `DRAW` and `BOX`. See `README-sprites.md` for why only eight things in
+this game can be made of artwork.