Selfo: show the selected piece and the cells it can reach

[fjvico]

Follow-up to #503 (which was merged before its last commit, so that one is included here).

• When a piece is selected, it is ringed and the empty cells it can move to are
  marked with dots. With the no-enclosure variant only the cells allowed by
  that rule are marked.
• move() now accepts {partial: true}. A half-entered move (just the piece)
  plays nothing and only sets the marks; validateMove sets canrender for
  that case and for the empty move, so the front end previews it.
• gameinfo.urls links the Wikipedia article on Selfo (the commit missing
  from Selfo: no colour starts one move away from winning #503).
• No translated strings are changed.

Tests: new "half-entered moves" block (marks match the legal moves with and
without no-enclosure, nothing is played, marks clear once a move is made,
complete moves flagged as partial still play, invalid selections still throw).
npm run build passes and the Selfo tests pass; checked by hand in the local
playground.