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>

tests/docs_preludes/akglbody.post

@@ -0,0 +1,2 @@
+    SUCCEED_RETURN(e);
+}

tests/docs_preludes/akglbody.pre

@@ -0,0 +1,21 @@
+/*
+ * Prelude for the akgl-backend fragment in docs/10-embedding.md. Same shape as
+ * hostbody.pre, plus the renderer and the two backend states those calls take.
+ * Only compiled in the AKBASIC_WITH_AKGL build, which is the only one where
+ * <akbasic/akgl.h> exists to include.
+ */
+#include <akerror.h>
+#include <akbasic/runtime.h>
+#include <akbasic/akgl.h>
+
+static akbasic_Runtime RUNTIME;
+static akbasic_GraphicsBackend graphics;
+static akbasic_SpriteBackend sprites;
+static akbasic_AkglGraphics gstate;
+static akbasic_AkglSprites sstate;
+static akgl_RenderBackend *my_renderer;
+
+akerr_ErrorContext AKERR_NOIGNORE *akbasic_docs_fragment(void);
+akerr_ErrorContext AKERR_NOIGNORE *akbasic_docs_fragment(void)
+{
+    PREPARE_ERROR(e);

tests/docs_preludes/hostbody.post

@@ -0,0 +1,2 @@
+    SUCCEED_RETURN(e);
+}

tests/docs_preludes/hostbody.pre

@@ -0,0 +1,22 @@
+/*
+ * Prelude for the statement fragments in docs/10-embedding.md: the variable
+ * exchange, the device lending, and the akgl backends.
+ *
+ * Each is shown as the two or three lines that matter, so they need a function
+ * to sit in and the surrounding declarations to refer to. `e` is the error
+ * context the chapter's examples name.
+ */
+#include <akerror.h>
+#include <akbasic/runtime.h>
+#include <akbasic/sink.h>
+
+static akbasic_Runtime RUNTIME;
+static akbasic_GraphicsBackend graphics;
+static akbasic_AudioBackend audio;
+static akbasic_InputBackend input;
+static akbasic_SpriteBackend sprites;
+
+akerr_ErrorContext AKERR_NOIGNORE *akbasic_docs_fragment(void);
+akerr_ErrorContext AKERR_NOIGNORE *akbasic_docs_fragment(void)
+{
+    PREPARE_ERROR(e);

tests/docs_preludes/hostcalls.post

@@ -0,0 +1,6 @@
+    } CLEANUP {
+    } PROCESS(errctx) {
+    } FINISH(errctx, true);
+    (void)score;
+    SUCCEED_RETURN(errctx);
+}

tests/docs_preludes/hostcalls.pre

@@ -0,0 +1,27 @@
+/*
+ * Prelude for the "used like this" fragment in README.md.
+ *
+ * The fragment is a run of CATCH calls, which only compile inside an ATTEMPT --
+ * that is the whole point of the macro protocol and the reason the fragment is
+ * shown without its scaffolding. `host_set_string` is the third accessor the
+ * surrounding prose says exists but does not print.
+ */
+#include <akerror.h>
+#include <akbasic/error.h>
+#include <akbasic/runtime.h>
+#include <akbasic/variable.h>
+
+static akbasic_Runtime SCRIPT;
+static const char *PROGRAM = "10 PRINT \"HI\"\n";
+
+akerr_ErrorContext AKERR_NOIGNORE *host_set_int(akbasic_Runtime *obj, const char *name, int64_t value);
+akerr_ErrorContext AKERR_NOIGNORE *host_get_int(akbasic_Runtime *obj, const char *name, int64_t *dest);
+akerr_ErrorContext AKERR_NOIGNORE *host_set_string(akbasic_Runtime *obj, const char *name, const char *value);
+
+akerr_ErrorContext AKERR_NOIGNORE *akbasic_docs_fragment(void);
+akerr_ErrorContext AKERR_NOIGNORE *akbasic_docs_fragment(void)
+{
+    PREPARE_ERROR(errctx);
+    int64_t score = 0;
+
+    ATTEMPT {

tests/docs_preludes/hostloop.post

@@ -0,0 +1,2 @@
+static int64_t your_clock_ms(void) { return 0; }
+static void your_draw_a_frame(void) { }

tests/docs_preludes/hostloop.pre

@@ -0,0 +1,13 @@
+/*
+ * Prelude for the "shortest useful host" example in docs/10-embedding.md.
+ *
+ * The example calls two functions it does not define, on purpose: a host's clock
+ * and a host's renderer are the host's business, and spelling them out would
+ * bury the four calls the section is actually about. Declare them here so the
+ * block compiles as written.
+ */
+#include <stdio.h>
+#include <akerror.h>
+
+static int64_t your_clock_ms(void);
+static void your_draw_a_frame(void);

tests/docs_preludes/hostvars.pre

@@ -0,0 +1,10 @@
+/*
+ * Prelude for the two host<->script accessors in README.md.
+ *
+ * The block is shown as two functions and nothing else, because the includes
+ * are already listed two sections above it.
+ */
+#include <akerror.h>
+#include <akbasic/error.h>
+#include <akbasic/runtime.h>
+#include <akbasic/variable.h>