fix(engine): keep composing when a reading reaches Maxrunes

At the limit the key that did not fit was handed back to the client as
plain ASCII, so a long Japanese reading ended in a Latin letter, and a
pending romaji consonant was flushed as ASCII with it (…あka for か).
Commit the completed kana, keep the pending syllable, and transliterate
the key as usual; other languages commit and go on the same way.

tests/engine_test.c

@@ -1904,7 +1904,7 @@ engine_randomized_stress(struct ct *t)
 void
 engine_full_boundary_passthrough(struct ct *t)
 {
-	Str com;
+	Str com, shown;
 	int i;
 
 	init();
@@ -1929,4 +1929,17 @@ engine_full_boundary_passthrough(struct ct *t)
 	CT_CHECK(t, keystroke(' ', 0, &com));
 	CT_EQ_INT(t, Maxrunes, com.n);
 	CT_EQ_UINT(t, ' ', com.r[Maxrunes-1]);
+
+	/* Typing on at the limit commits the kana and keeps the syllable. */
+	init();
+	im.l = getlang(LangJP);
+	for(i = 0; i < Maxrunes-1; i++)
+		im.pre.r[i] = L'あ';
+	im.pre.n = Maxrunes-1;
+	im.raw = mkstr("k");
+	sclear(&com);
+	CT_CHECK(t, keystroke('a', 0, &com));
+	CT_EQ_INT(t, Maxrunes-1, com.n);
+	impre(&im, &shown);
+	checkstr(t, "syllable after the flush", "か", &shown);
 }