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/10-embedding.md

@@ -27,7 +27,7 @@ target_link_libraries(YOUR_GAME PRIVATE akbasic::akbasic)
 
 ## The shortest useful host
 
-```c
+```c wrap=hostloop
 #include <akbasic/runtime.h>
 #include <akbasic/sink.h>
 
@@ -65,7 +65,7 @@ immediately — audible, but never a hang.
 Use `akbasic_runtime_global()`. It finds or creates the variable in the script's
 outermost scope, which is the only place both of you can reliably see:
 
-```c
+```c wrap=hostbody
 akbasic_Variable *health = NULL;
 int64_t subscript[1] = { 0 };
 
@@ -80,7 +80,7 @@ it stops.
 
 ## Lending devices
 
-```c
+```c wrap=hostbody
 PASS(e, akbasic_runtime_set_devices(&RUNTIME, &graphics, &audio, &input, &sprites));
 ```
 
@@ -91,7 +91,7 @@ all.
 
 `akbasic_akgl` provides implementations that draw through a renderer *you* created:
 
-```c
+```c wrap=akglbody requires=akgl
 PASS(e, akbasic_graphics_init_akgl(&graphics, &gstate, my_renderer));
 PASS(e, akbasic_sprite_init_akgl(&sprites, &sstate, my_renderer, &gstate));
 ```