engine: a key that converts nothing is the application's

Space and Tab convert a Japanese reading, and a reading with no
candidate to step through was committed and the key eaten.  Katakana
mode converts nothing -- katakouho adds the Katakana form only when it
differs from the reading, and in Katakana mode it never does -- so
every Space between two Katakana words was swallowed and had to be
typed twice: カク<Space> committed カク and left no space, and the
second Space passed through only because nothing was pending any more.

Committing and passing the key on gives the space back.  Hiragana is
unchanged wherever the reading has kana, since the Katakana form is
always one candidate there; it changes only for a reading that made no
kana at all, かx<Space>, which now commits and spaces too.

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

README.md

@@ -41,7 +41,8 @@ kana shown, and romaji that made no kana stays as typed until it is mended;
 The dictionary keys verbs and adjectives by stem and okurigana, so `kaku`
 offers かく's readings and then 書く and its kin — typing the okurigana
 with `Shift`, as SKK does, `kaKu`, asks for that split first. Katakana
-mode does not convert.
+mode does not convert, so there `Space` and `Tab` commit the reading and
+go on to the application.
 
 Korean composes one syllable at a time. `Enter`, `Tab`, `Esc` and any key
 that is not a jamo commit the syllable and go on to the application, so