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

docs/10-embedding.md

@@ -100,6 +100,50 @@ bounded run is usually inside a `FOR` or `GOSUB` body, and a variable created th
 dies when the body pops — silently, with the script reading it correctly right up until
 it stops.
 
+## Calling a function every frame
+
+`akbasic_runtime_call_function()` calls a `DEF` by name with values you already
+hold — the entry point a game loop wants. A host that calls it repeatedly signs
+up for three rules the one-shot examples never meet:
+
+```c wrap=hostcalls
+CATCH(errctx, akbasic_runtime_call_function(&SCRIPT, "THINK", argp, 1, &result));
+/* ...consume the result... */
+CATCH(errctx, akbasic_environment_zero(SCRIPT.environment));
+```
+
+1. **Reset the value scratch after every call, once the result is consumed.**
+   Each call parks its result in the caller environment's per-line scratch
+   (`AKBASIC_MAX_VALUES` slots), and a host calling in a loop never crosses the
+   line boundary that would reset it. Skip the `akbasic_environment_zero()` and
+   the pool drains — measured at under two frames of forty calls — after which
+   every call fails with `Maximum values per line reached`. The reset also
+   invalidates `result`, which is why it comes after the consumption.
+2. **Force RUN mode once after the boot run.** A multi-line `DEF` body only
+   runs while the runtime is in RUN mode, and by the time a host can call, the
+   program that filed the definitions has ended. One
+   `akbasic_runtime_set_mode(&SCRIPT, AKBASIC_MODE_RUN)` after
+   `akbasic_runtime_run()` makes the bodies run, and the mode stays put because
+   nothing steps the runtime between calls. Issue #8 tracks making this
+   unnecessary.
+3. **Revive after a script error, deliberately.** A BASIC-level error inside a
+   called body reports through the sink, answers a stale value, and latches:
+   the runtime leaves RUN mode and every later call does nothing. When your
+   policy is to absorb the error and keep calling — a game marking one actor
+   dumb rather than killing the frame — the revival is two calls:
+   `akbasic_runtime_clear_error()`, then `akbasic_runtime_set_mode(RUN)` again.
+   The latch is deliberate for *programs* — the first error ends a run, once,
+   with one line — so nothing clears it for you.
+
+Do not pass structures as per-frame arguments. A structure or pointer parameter
+spends a value-pool slot on every call and the pool never reclaims, so the
+interface dies after about a thousand calls — issue #36 has the measurements.
+Bind the instance once with `akbasic_host_bind()` and point it at each object
+with `akbasic_host_rebind()` ([Chapter 16](16-structures.md)), which spends
+nothing per call. The GALAGA tutorial ([Chapters 20](20-tutorial-galaga.md)
+and [21](21-tutorial-galaga-enemies.md)) is this whole recipe as a working
+game, forty calls a frame.
+
 ## Where the output goes
 
 `PRINT` writes through an `akbasic_TextSink`, which is a record of function pointers plus