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/07-sound.md

@@ -6,7 +6,7 @@ name, and a machine with no sound card still runs the interpreter — only `SOUN
 
 ## SOUND
 
-```
+```basic requires=akgl
 10 SOUND 1, 4000, 60
 ```
 
@@ -24,7 +24,7 @@ frame loop for a second would freeze the game.
 
 `PLAY` takes a string of notes:
 
-```
+```basic requires=akgl
 10 PLAY "C D E F G"
 ```
 
@@ -50,7 +50,7 @@ immediately `QUIT`s may not hear all of it.
 
 ## TEMPO
 
-```
+```basic requires=akgl
 10 TEMPO 8
 ```
 
@@ -60,7 +60,7 @@ transcription, so it may not match a real machine exactly.
 
 ## ENVELOPE and VOL
 
-```
+```basic requires=akgl
 10 ENVELOPE 0, 5, 9, 4, 6
 20 VOL 8
 ```
@@ -72,8 +72,13 @@ and release. `VOL` sets the overall volume, 0 to 15.
 
 `FILTER` parses and then **refuses at execution**:
 
+```basic
+10 FILTER 1000, 1, 0, 0, 8
 ```
-? 10 : RUNTIME ERROR FILTER needs an audio device that can filter, and this one cannot
+
+```output
+? 10 : RUNTIME ERROR FILTER needs a filter stage this device does not have
+
 ```
 
 It sets the SID's filter cutoff, band switches and resonance. The audio backend