engine: Backspace gives the reach back before what was typed

searchkey popped search.raw and then search.seed but never touched
search.back, so once the typed syllable was gone the query was nothing
but the client's own text: an empty preedit above a full candidate list
drawn from syllables the user had not selected, and Enter rewrote them.
With 상 written and 태 typed, Ctrl+H offered 狀態; one Backspace left
the query 상 and 128 candidates, and Enter replaced the 상 with 上 and
dropped the 태 altogether.

Undo now runs backwards through what happened -- the keys typed since
Ctrl+H, then the reach Ctrl+H made, then the syllable that seeded it --
so the same Backspace narrows 상태 to 태 and offers 太, which is also
the only way there was ever going to be to convert the syllable alone.

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

strans.c

@@ -848,6 +848,8 @@ searchkey(u32int ks, u32int mod, Str *com)
 	if(ks == Kback){
 		if(search.raw.n != 0)
 			spopr(&search.raw);
+		else if(search.back.n != 0)
+			sclear(&search.back);	/* the reach undone, in its turn */
 		else if(search.seed.n != 0)
 			spopr(&search.seed);
 		else{