Keep what a program draws, instead of making it a sprite

A drawing lasted exactly one frame. The verbs are immediate, they went to the
back buffer, SDL double-buffers and the frontend never clears -- so the only way
to keep a picture was to capture it with `SSHAPE` and install it as a sprite,
which is what `examples/breakout/sprites/breakout.bas` spends two of its eight
sprites doing. That was TODO.md section 9 item 9.

The drawing verbs now render into a layer texture the frame composites under the
text and the sprites. Draw once; it is there on every frame after.

**Bracketed around the step phase, not around each verb.** One pair of
`SDL_SetRenderTarget` calls a frame instead of one per `DRAW`, and it is also
what makes `SSHAPE` read back what the program has just drawn rather than
whatever the last frame left.

**The layer is transparent where nothing was drawn.** It covers the whole window
and composites underneath, so an opaque one would black out the frame the moment
a program issued a single `DRAW`. And a fresh SDL target texture's contents are
undefined, so it is cleared on creation -- skipping that puts uninitialised
memory under the first frame's text and looks like a driver bug rather than a
missing memset.

**The line editor forced a wrinkle worth naming.** `akbasic_frontend_akgl_pump()`
is called from two places with different answers to "is a render target current":
the frame loop calls it between steps, and the sink's editor calls it from
*inside* a step, borrowing a frame while it waits for a typed line. SDL refuses
to present while a target is current, so the pump ends the layer, presents, and
puts it back only if it was the one that ended it. `akgl_frontend` caught this --
it drives a REPL session, and it failed with "You can't present on a render
target" the first time the brackets went in.

This does not make a drawing *visible* on its own. The text layer still repaints
every row it owns, opaque, every frame, and by default it owns the whole window;
`WINDOW` shrinks it and that half was already fixed. The two together are what a
picture needed, and the tests assert both -- a pixel still there a frame later
with nothing redrawn, and a pixel below a shrunk text area surviving the text
repaint. The second assertion wipes to a non-black colour first, because against
black it could not tell a transparent layer from an opaque one.

The tests found two of their own bugs on the way: `stop_runtime()` was not
tearing the graphics backend down, so re-initialising it dropped a live texture
on the floor; and a first draft called `begin()` before `start_runtime()`, which
re-inits the backend, so the assertion read back off an orphaned render target
and passed while proving nothing.

Chapters 6 and 13 stop saying a drawing has to be redrawn every frame, because it
does not. The batch-boundary tear stays documented -- it bites an `SSHAPE`
capture, which matters much less now that capturing is not the only way to keep a
picture.

Both games still run clean. 111 with akgl, 110 without.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EwxGB6TdoVvZ11KQQME9cL

docs/06-graphics.md

@@ -240,10 +240,23 @@ does with one, but you cannot store it or measure it.
 
 `FILTER` parses and then refuses: there is no filter stage to configure. See Chapter 7.
 
-The graphics verbs draw straight to the renderer rather than into a display list, so
-anything drawn is overwritten by the text layer on the next frame. A program that wants
-its drawing to persist has to redraw it. This is recorded as a defect rather than a
-design; see Chapter 13.
+**A drawing stays.** The verbs render into a layer the frame composites underneath the
+text and the sprites, so a program draws its picture once and it is there on every frame
+after. It does not have to redraw it, and it does not have to capture it into a sprite.
+
+What covers it is the text layer, which repaints every row it owns — opaque, every frame,
+for a reason `akbasic_sink_akgl_render()` explains — and by default it owns the whole
+window. `WINDOW` shrinks it:
+
+```basic norun
+10 WINDOW 0, 0, 39, 1
+20 GRAPHIC 1, 1
+30 COLOR 1, 3
+40 BOX 1, 20, 40, 300, 180
+```
+
+Two rows of text at the top, the rest of the window for drawing, and the box is still
+there a thousand frames later.
 
 **A redraw also has to fit inside one batch.** The host runs a fixed number of source
 lines and then presents, and presenting throws the drawing buffer away — so a run of