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.

strans.c

@@ -751,9 +751,13 @@ transition(u32int ks, u32int mod, Str *com)
 		return 0;
 	if(im.pre.n >= Maxrunes ||
 	   (isjp(&im) && im.pre.n + im.raw.n >= Maxrunes)){
-		commit(com);
-		reset();
-		return 0;
+		/* A full reading commits; a pending romaji syllable stays. */
+		if(isjp(&im))
+			sappend(com, &im.pre);
+		else
+			commit(com);
+		sclear(&im.pre);
+		clearkouho();
 	}
 	return dotrans(ks, com);
 }