engine: Space offers the reading in Katakana last; 0 always commits the reading

Every Japanese IME converts a reading to Katakana on demand; strans made
the user switch to the Katakana mode and type again.  Space now adds
the reading in Katakana after the dictionary candidates, so a loanword
the dictionary lacks converts with one Space and Enter, and 漢字 users
still get 漢字 first.

0 committed the reading only while candidates showed; without them it
typed a 0 as well.  It now commits any Japanese reading, the counterpart
of 1-9 picking candidates, as the README always said.

README.md

@@ -26,8 +26,9 @@ unmodified `1`-`9` commits that row of the current page. `Enter` and `Tab`
 commit 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.
 Japanese consumes the confirming key; Korean and Vietnamese pass it on to
-the application. `Space` on a reading without candidates commits it and
-types nothing. `Backspace` and `Esc` on a chosen candidate go back to the
+the application. `Space` adds the reading in Katakana as the last
+candidate, so a word the dictionary lacks converts to Katakana with one
+`Space`. `Backspace` and `Esc` on a chosen candidate go back to the
 reading. In a temporary Emoji or Hanja search, `Enter` commits the
 highlighted result and `Tab`/`Shift-Tab` wrap through the results. Leaving
 the field or clicking elsewhere commits what is pending. `Esc` cancels a