Write the GALAGA tutorial chapters and the repeated-host-calls guide

docs/20 builds the engine and the boundary: the startup order, the
starfield, actors and collision, booting a DEF-only script, the issue #8
mode workaround, the custom update hook, first light, screens, and the
headless harness. docs/21 builds the three shared structures and the AI:
the host type tables, the actor binding, the randomness route around
issue #16, the measured case against structure arguments (issue #36),
the three language rules that shape the script, the maneuvers, the
argued formation decision, the script-death policy, and the interop
proof. Every fenced block runs under tests/docs_examples.sh in both
build configurations; five new preludes carry the C fragments.

docs/10 gains the 'Calling a function every frame' section the chapters
lean on: the per-call akbasic_environment_zero() rule, the set_mode(RUN)
workaround, the clear_error() revival, and the case for rebinding over
structure arguments. Index rows and chapter counts updated.

Co-authored-by: andrew <andrew@aklabs.net>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XiGgpHuXUm2mR4Wzndw3dc

tests/docs_preludes/galagacalls.pre

@@ -0,0 +1,53 @@
+/*
+ * Prelude for the interpreter-facing fragments in docs/20: the boot sequence,
+ * the ADDEM proof and the rebind-call-reset protocol, shown as runs of CATCH
+ * calls. The statics are the ones examples/galaga/script.c keeps; the locals
+ * are the superset every fragment draws from, void-cast in the postlude so an
+ * unused one is not a warning.
+ */
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <string.h>
+
+#include <SDL3/SDL.h>
+
+#include <akerror.h>
+#include <akstdlib.h>
+
+#include <akgl/actor.h>
+
+#include <akbasic/environment.h>
+#include <akbasic/error.h>
+#include <akbasic/host.h>
+#include <akbasic/runtime.h>
+#include <akbasic/sink.h>
+
+typedef struct galaga_docs_Enemy
+{
+    int32_t  kind;
+    int32_t  state;
+    float    homex;
+    float    homey;
+    float    t;
+    int32_t  hp;
+    int32_t  fire;
+    float    rnd;
+} galaga_docs_Enemy;
+
+static akbasic_Runtime SCRIPT;
+static akbasic_TextSink SINK;
+static akbasic_StdioSink SINKSTATE;
+static char SOURCE[16384];
+
+akerr_ErrorContext AKERR_NOIGNORE *galaga_docs_fragment(galaga_docs_Enemy *enemy, akgl_Actor *actor, float dt);
+akerr_ErrorContext AKERR_NOIGNORE *galaga_docs_fragment(galaga_docs_Enemy *enemy, akgl_Actor *actor, float dt)
+{
+    PREPARE_ERROR(errctx);
+    akbasic_Value args[2];
+    akbasic_Value *argp[2];
+    akbasic_Value dtval;
+    akbasic_Value *result = NULL;
+
+    ATTEMPT {