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/01-introduction.md

@@ -33,7 +33,7 @@ The default build has no dependency on SDL and no graphics, sound, sprites or wi
 text. Everything else works, and the whole test suite runs on a machine with no SDL
 installed at all.
 
-```sh
+```sh norun
 cmake -S . -B build
 cmake --build build
 ```
@@ -42,7 +42,7 @@ The SDL build adds a window, the Commodore font, and the graphics, sound and spr
 devices. It needs [libakgl](https://source.starfort.tech/andrew/libakgl), which is
 vendored as a submodule.
 
-```sh
+```sh norun
 git submodule update --init --recursive
 cmake -S . -B build-akgl -DAKBASIC_WITH_AKGL=ON
 cmake --build build-akgl
@@ -51,8 +51,13 @@ cmake --build build-akgl
 A verb that needs a device the build does not have does not crash and does not lie. It
 reports itself 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
+
 ```
 
 That is the same message an embedded host sees when it deliberately withholds a
@@ -60,7 +65,7 @@ device — a game may want a script that can print but not draw.
 
 ## Running the tests
 
-```sh
+```sh norun
 ctest --test-dir build --output-on-failure
 ```