La Rondine  Peacock Theatre, London 3 out of 5 3  Ninety years
after its first performance, Puccini's lyric comedy still only
hovers on the fringes of the repertory. Part of the reason is an
unsatisfactory final act, which the perfectionist composer
attempted to fix in a couple of major revisions. Yet opera
companies tend to stick to the original version, as British
Youth Opera does here.    1. La Rondine   2. Peacock Theatre,
3. London   1. Final performances tonight and Thursday. Box
office: 0844 412 4322.  Set in Second Empire Paris - updated in
John Lloyd Davies's production by 50 years or so - La Rondine
(the swallow) describes how kept woman Magda leaves the lap of
courtesan luxury to run off to the Riviera with the penniless
Ruggero. In the last act, they hit the twin buffers of
conventional morality and financial reality, and Magda returns
to her former life.  Though the ending does not convince either
dramatically or musically, the first two acts, set in Magda's
salon where the lovers meet and the