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/07-sound.md

@@ -13,6 +13,12 @@ name, and a machine with no sound card still runs the interpreter — only `SOUN
 Voice, frequency, duration. Voices are numbered from 1. The duration is in *jiffies* —
 sixtieths of a second — as on a C128, so 60 is one second.
 
+**The frequency is a SID register value, not hertz.** As on a C128 the pitch is
+`frequency * 1022730 / 16777216` — the NTSC system clock over the oscillator's 24-bit
+accumulator — so the 4000 above is about 244 Hz, and the argument ranges from 0 to
+65535. Work the notes you need out once and write the arithmetic down beside them: 4298
+is C4, 8579 is C5, 17175 is C6. `PLAY` takes note names and does the conversion for you.
+
 Further arguments give a frequency sweep: a step, a direction and a range, which is
 what makes a siren or a laser.