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/galagahost.pre

@@ -0,0 +1,55 @@
+/*
+ * Prelude for statement-context fragments in docs/20: runs of CATCH calls
+ * from the galaga frame loop, shown without their scaffolding because the
+ * ATTEMPT protocol is the scaffolding. Same policy as hostcalls.pre.
+ */
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <string.h>
+
+#include <SDL3/SDL.h>
+#include <SDL3_ttf/SDL_ttf.h>
+
+#include <akerror.h>
+#include <akstdlib.h>
+
+#include <akgl/actor.h>
+#include <akgl/controller.h>
+#include <akgl/draw.h>
+#include <akgl/game.h>
+#include <akgl/heap.h>
+#include <akgl/renderer.h>
+#include <akgl/ui.h>
+
+#include <akbasic/error.h>
+#include <akbasic/runtime.h>
+
+typedef enum
+{
+    GALAGA_SCREEN_TITLE = 0,
+    GALAGA_SCREEN_PLAY,
+    GALAGA_SCREEN_GAMEOVER,
+    GALAGA_SCREEN_VICTORY
+} galaga_Screen;
+
+struct galaga_docs_Game
+{
+    galaga_Screen screen;
+    float dt;
+    akgl_Actor *player;
+};
+extern struct galaga_docs_Game galaga_game;
+extern akgl_Actor *galaga_enemy_actors[40];
+
+akerr_ErrorContext AKERR_NOIGNORE *declare_title(void);
+akerr_ErrorContext AKERR_NOIGNORE *declare_play(void);
+akerr_ErrorContext AKERR_NOIGNORE *declare_end(void);
+akerr_ErrorContext AKERR_NOIGNORE *galaga_docs_fragment(void);
+akerr_ErrorContext AKERR_NOIGNORE *galaga_docs_fragment(void)
+{
+    PREPARE_ERROR(errctx);
+    SDL_Event event;
+
+    ATTEMPT {