Forward WINDOW through the tee sink

`akbasic_sink_init_tee()` wired write, writeln, readline, clear and --
conditionally -- moveto, and never `window`. The standalone AKGL frontend runs
the interpreter against a tee, so the fully implemented `sink_window()`
underneath it was unreachable: `WINDOW 0, 0, 20, 4` answered "WINDOW needs a
text device with a character grid, and this one has none" on the one build that
has a character grid. It reads as an oversight rather than a decision, because
`moveto` directly above it is forwarded with reasoning that applies unchanged.

`tee_window()` is that function, modelled on `tee_moveto()`, offered only when a
half can take it so the refusal still reads correctly through a stdio-only pair.

**Writing the test found a second one.** Both optional entry points are assigned
conditionally, and neither initializer cleared them first -- so a caller with a
sink on the stack got whatever was in that memory, and `CHAR` and `WINDOW`
decide whether they can act by testing those pointers for NULL. Both
`akbasic_sink_init_tee()` and `akbasic_sink_init_stdio()` now clear them. An
initializer that leaves a field alone is not an initializer.

tests/sink_tee.c gains a stand-in sink with a grid -- the two stdio halves have
neither entry point, which is exactly why they could not show that either is
forwarded -- and asserts both directions, the arguments arriving intact, and
that a grid-less pair still offers neither.

Verified end to end: `WINDOW 0, 0, 20, 4` then `PRINT` succeeds under
build-akgl/basic and still refuses by name under build/basic.

What this does *not* fix is that a drawing still has to be re-issued every
frame: the frontend never clears and SDL is double-buffered, so a drawing issued
once appears in one buffer only. Shrinking the text area makes the rest of the
window the program's; keeping something there is still the program's job. That
half is documented rather than fixed, and is filed as TODO.md section 9 item 5.

TODO.md section 6 item 31, struck.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/sink_stdio.c

@@ -100,5 +100,13 @@ akerr_ErrorContext *akbasic_sink_init_stdio(akbasic_TextSink *obj, akbasic_Stdio
     obj->writeln = stdio_writeln;
     obj->readline = stdio_readline;
     obj->clear = stdio_clear;
+    /*
+     * The optional entry points, cleared rather than left alone. A stream has no
+     * character grid, so `CHAR` and `WINDOW` must refuse by name -- and they
+     * decide that by testing the pointer for NULL. A caller with a sink on the
+     * stack would otherwise hand them whatever was in that memory.
+     */
+    obj->moveto = NULL;
+    obj->window = NULL;
     SUCCEED_RETURN(errctx);
 }