Execute every documented example as a test

docs/ and README.md carry 85 fenced blocks. Every one was checked by hand
exactly once, when it was written, which is not a standard that survives a
changing interpreter -- and four were already wrong: two transcripts showing a
leading space PRINT does not emit, akbasic_TextSink in README.md missing the
two members it had grown hours earlier, and FILTER's refusal quoted with
wording the code does not use.

tests/docs_examples.sh reads a fence-tag vocabulary and runs what it finds.
BASIC programs and transcripts run and are byte-compared against an `output`
block; C snippets compile with -fsyntax-only against the real include path,
which CMake writes out because it is transitive through akerror, akstdlib and
akgl; shell blocks run in a sandbox. Anything that would reconfigure the build
tree, hit the network or re-enter the suite is tagged norun with the reason in
MAINTENANCE.md, and the two cmake blocks stay hand-maintained by decision.

An untagged block is a failure rather than a default, and the pass line
reports what it executed by kind. Both exist because the way a harness like
this dies is by quietly matching nothing and passing -- which it duly did on
the first CTest run, where a generator expression evaluating to nothing still
contributed an empty argument that the script read as a filename. The count is
what caught it.

The excerpt check earns its own mention: a block tagged
`c excerpt=include/akbasic/sink.h` must still appear in that header, comments
and whitespace ignored. Compiling it would only redefine the type, so a
compile check could not have found the stale struct, and did not.

Registered as the CTest case docs_examples in both configurations. Fixing the
four wrong examples turned up two interpreter defects, fixed in the previous
commit and recorded in TODO.md section 8.

MAINTENANCE.md is new: the fence-tag reference, what to do when the case
fails, and the conventions that until now only existed inside source comments
-- the three test lists and how two of them invert "passed", the sorted verb
table, that a golden file is never edited to suit this interpreter, and that a
fix gets mutation-checked with a file copy rather than git checkout.

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

docs/06-graphics.md

@@ -3,8 +3,13 @@
 Everything in this chapter needs the SDL build and a graphics device. Without one each
 verb refuses by name:
 
+```basic requires=noakgl
+10 DRAW 1, 0, 0 TO 100, 100
 ```
+
+```output
 ? 10 : RUNTIME ERROR DRAW needs a graphics device and this runtime has none
+
 ```
 
 ## The coordinate space
@@ -19,7 +24,7 @@ screen.
 `COLOR` binds a *source* to a palette index, and the drawing verbs name the source
 rather than the colour:
 
-```
+```basic requires=akgl
 10 COLOR 1, 3
 20 DRAW 1, 10, 20
 ```
@@ -37,7 +42,7 @@ refuses to draw.
 
 ### DRAW
 
-```
+```basic requires=akgl
 10 DRAW 1, 10, 20
 20 DRAW 1, 0, 0 TO 100, 100 TO 200, 0
 ```
@@ -47,7 +52,7 @@ bare `DRAW 1` plots wherever `LOCATE` left the pixel cursor.
 
 ### BOX
 
-```
+```basic requires=akgl
 10 BOX 1, 10, 10, 40, 40
 ```
 
@@ -55,7 +60,7 @@ Corners, and an optional rotation angle. An unrotated `BOX` outlines rather than
 
 ### CIRCLE
 
-```
+```basic requires=akgl
 10 CIRCLE 1, 160, 100, 50, 30
 ```
 
@@ -65,7 +70,7 @@ an arc or a polygon.
 
 ### PAINT
 
-```
+```basic requires=akgl
 10 PAINT 1, 160, 100
 ```
 
@@ -80,7 +85,7 @@ coordinates finishes.
 
 ### SCALE
 
-```
+```basic requires=akgl
 10 SCALE 1, 1023, 1023
 ```
 
@@ -96,7 +101,7 @@ parallel passes; see Chapter 13.
 
 `SSHAPE` copies a rectangle off the screen and `GSHAPE` stamps it back:
 
-```
+```basic requires=akgl
 10 BOX 1, 0, 0, 20, 20
 20 SSHAPE A$, 0, 0, 20, 20
 30 GSHAPE A$, 100, 100