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/05-strings-and-formatting.md

@@ -22,7 +22,7 @@ string has gives you the whole string.
 `MID("HELLO WORLD", 6, 5)` is `WORLD`, and `INSTR("HELLO WORLD", "WORLD")` is `6`. A
 failed `INSTR` gives -1, not 0, so there is no ambiguity with a match at the start.
 
-```
+```basic
 10 A$ = "HELLO WORLD"
 20 PRINT LEN(A$)
 30 PRINT LEFT(A$, 5)
@@ -31,6 +31,14 @@ failed `INSTR` gives -1, not 0, so there is no ambiguity with a match at the sta
 60 PRINT INSTR(A$, "WORLD")
 ```
 
+```output
+11
+HELLO
+WORLD
+WORLD
+6
+```
+
 `VAL` refuses text that is not a number rather than quietly returning zero, so a
 program can tell "the user typed 0" from "the user typed nonsense".
 
@@ -38,13 +46,13 @@ program can tell "the user typed 0" from "the user typed nonsense".
 
 `PRINT USING` lays a value out in a fixed field, which is what makes columns line up.
 
-```
+```basic
 10 PRINT USING "###.##"; 3.14159
 20 PRINT USING "TOTAL: $#,###.##"; 1234.5
 30 PRINT USING "####-"; -42
 ```
 
-```
+```output
   3.14
 TOTAL: $1,234.50
   42-
@@ -68,11 +76,11 @@ stands:
 is loud: printing more digits than the field asked for would push every later column
 out of line.
 
-```
+```basic
 10 PRINT USING "###"; 99999
 ```
 
-```
+```output
 ***
 ```
 
@@ -81,12 +89,12 @@ printing `-5` as `5` would be worse.
 
 String fields centre and right-justify:
 
-```
+```basic
 10 PRINT USING "=========="; "MID"
 20 PRINT USING ">>>>>>>>>>"; "RIGHT"
 ```
 
-```
+```output
    MID    
      RIGHT
 ```
@@ -100,12 +108,12 @@ Chapter 13.
 to four, in this order: the leading blank, the thousands separator, the decimal point,
 and the currency sign.
 
-```
+```basic
 10 PUDEF "*"
 20 PRINT USING "#####"; 42
 ```
 
-```
+```output
 ***42
 ```
 
@@ -116,11 +124,17 @@ only the padding.
 
 `CHAR` puts text at a character position rather than at the cursor:
 
-```
+```basic requires=akgl
 10 CHAR 1, 10, 5, "HERE"
+20 PRINT
 ```
 
-The arguments are colour, column, row and the text. It needs a text device with a
+```output
+HERE
+```
+
+The arguments are colour, column, row and the text. `CHAR` writes no newline of its
+own and leaves the cursor after the text, which is why line 20 is there. It needs a text device with a
 cursor, which means the SDL build — a terminal's cursor is not this library's to move,
 and it refuses by name in the default build.