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/hostcalls.pre

@@ -7,6 +7,7 @@
  * surrounding prose says exists but does not print.
  */
 #include <akerror.h>
+#include <akbasic/environment.h>
 #include <akbasic/error.h>
 #include <akbasic/runtime.h>
 #include <akbasic/variable.h>
@@ -23,5 +24,7 @@ akerr_ErrorContext AKERR_NOIGNORE *akbasic_docs_fragment(void)
 {
     PREPARE_ERROR(errctx);
     int64_t score = 0;
+    akbasic_Value *argp[4];
+    akbasic_Value *result = NULL;
 
     ATTEMPT {