Units, orders, and finding the water under an army

Convoy routing is breadth-first over the fleets actually ordered to convoy
this army, so it returns the shortest chain and answers null rather than
guessing. It takes a set of fleets to treat as already gone, which is what
the adjudicator will need to ask whether a convoy survives being attacked.

The tests found a wrong assumption of mine rather than a wrong line of code:
taking the Channel out of a London to Brest convoy leaves no route at all.
The North Sea does not touch the Mid-Atlantic on this board -- the long way
round is over the top through the Norwegian Sea -- so the Channel is the
whole western sea route.

README.md

@@ -39,10 +39,21 @@ would not be.
 other three games use, on a map generated from the topology in
 `src/game/map.ts` rather than traced from anybody's board.
 
+**It gets the whole screen.** The other three games live in a four-by-three
+cabinet because the machines they are rebuilding did. This one is not
+rebuilding a machine, and seventy-five provinces plus somebody trying to talk
+you out of Galicia do not fit in a letterbox. The map takes the viewport.
+
 **The music is not cartoony, and that is the point.** It is 1914: serious,
 and intense. The joke of this game is that something charming to look at is
 the cruellest thing on the site, and the score is what says so.
 
+**And the turn should be heard.** Orders resolve in silence in every online
+version of this game, which is a waste of the one moment anybody cares
+about. Armies grinding into each other, guns at sea, a convoy escorting
+something across, a unit being built, a unit falling back -- each of those is
+a different noise, generated by the synth like everything else on this site.
+
 ## The board
 
 Seventy-five provinces, two adjacency graphs — an army walks the land, a