engine: the digits type until the candidate list is engaged

Every complete reading shows its candidates, so 1-9 were candidate picks
in the middle of composing: ka then 5 committed 家, and heya then 2
typed a 2 the application never asked for.  They pick only once Space,
Tab, or an arrow has chosen a candidate, as in Mozc; before that they are
ordinary keys.  A search is unchanged: its digits pick from the start.

README.md

@@ -31,8 +31,8 @@ adjectives SKK keys by stem and okurigana (かく offers 確 and 書く), and
 under `Space` the reading in Katakana last, so a word the dictionary lacks
 converts to Katakana with one `Space`. `Space` or `Tab` chooses the first
 candidate and then steps on, wrapping (with `Shift`, back); `Up`/`Down` and
-`PageUp`/`PageDown` move without wrapping. An unmodified `1`-`9` commits
-that row of the current page. `Enter` commits the chosen candidate, or the
+`PageUp`/`PageDown` move without wrapping. Once a candidate is chosen, an unmodified `1`-`9` commits that row of the
+current page; before that the digits type. `Enter` commits the chosen candidate, or the
 reading when none is chosen, as `0` always does; so does typing on, or any
 key that ends the composition. `Backspace` deletes the last kana shown;
 romaji that made no kana stays in the reading as typed until it is mended.