Add Contract Bridge: auction, dummy play, and rubber scoring

New cg-bridge.el: a four-handed Bridge game (you are South, partnering
North against East and West) on the shared cg-game base.

* Auction: level/strain bids plus pass, double, and redouble, with the
  three-pass end rule, pass-outs, doubling state, and declarer
  determination (first of the side to name the strain).
* Play: follow-suit with the dummy exposed after the opening lead; the
  declarer plays both hands. Trick resolution honours trump and no-trump.
* Scoring: classic rubber -- trick points below the line toward game;
  overtricks, slam, insult, and undertrick penalties above; vulnerability
  and the rubber bonus. Verified against known results.
* A small natural bidding AI (openings, NT, raises with a fit, simple
  overcalls) that always terminates the auction, plus a greedy
  card-play AI.

Wire cg-bridge into the chooser, the Makefile, and the README, and add
two ERT tests (scoring math and a dozen full AI-driven deals). The suite
is now 109/109 and every file byte-compiles cleanly.

README.org

@@ -105,6 +105,12 @@ with its command.
 - ~cg-spite~ -- Spite & Malice.  Race the computer to empty your goal
   pile onto shared centre piles that build Ace to Queen; Kings are wild.
 
+** Bridge
+- ~cg-bridge~ -- Contract Bridge.  A full auction (bids, pass, double,
+  redouble), play with the dummy exposed, and classic rubber scoring with
+  vulnerability.  You are South; when you declare you play the dummy too.
+  The bidding AI is a small natural system, sensible but no expert.
+
 * TODO
 - [X] make the suit symbols customizable (~cg-symbols~) and obey them
 - [ ] a Texinfo manual