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>

docs/README.md

@@ -13,6 +13,10 @@ everything that behaves differently here and why. **[Chapter 14](14-architecture
 the odd one out: it is about the interpreter rather than the language, for anyone
 embedding it, debugging it or changing it.
 
+**[Chapters 17](17-tutorial-breakout.md)** and **[18](18-tutorial-breakout-artwork.md)**
+are tutorials rather than reference: they build one complete game twice, two different
+ways, and are where the rules that a real program runs into are collected.
+
 ## Chapters
 
 | | |
@@ -33,6 +37,8 @@ embedding it, debugging it or changing it.
 | **[14. Architecture](14-architecture.md)** | How the interpreter is put together, how to debug it, how to change it |
 | **[15. Error codes](15-error-codes.md)** | Appendix: every value `ER#` can hold and every error line the interpreter prints |
 | **[16. Structures](16-structures.md)** | `TYPE`, records, strict pointers, and sharing a C struct with an embedding host |
+| **[17. Tutorial: Breakout](17-tutorial-breakout.md)** | Build a whole game out of the text grid and two `DATA` sprites, a step at a time |
+| **[18. Tutorial: Breakout with artwork](18-tutorial-breakout-artwork.md)** | Build it again out of loaded artwork, powerups and a drawn HUD |
 
 ## The shortest possible start