Convoys were in the rules and not on the board

Fifty of the DATC cases are about convoys, paradoxes included, and the
adjudicator passes them. But the army's half of a convoy could not be
ordered: the board offered a unit the provinces it could walk to, and a
crossing is by definition not one of those. The fleet's half could be
ordered, which made it worse -- the verb was there and did nothing anybody
could reach.

An army on a coast is now also offered the coasts a chain of crewed seas can
reach, in blue with long dashes, so a crossing does not look like a march. A
sea counts if a fleet is standing in it, whoever owns it; a fleet that has
not been ordered to convoy still counts, because it is a thing that could be
arranged and that is what the talking is for.

This is deliberately not the question the rules ask when they judge a convoy
order, which is whether water could ever get there. That one says yes to
most of Europe: offering thirty provinces because a chain of fleets is
conceivable is worse than offering none.

Clicking a blue coast writes the move viaConvoy. Not a formality -- the
rules let a unit be convoyed to a province it could have walked to, and the
two orders resolve differently.

The fleet's own offering was too loose in the other direction and is now on
the same chain: the North Sea listed eleven armies it could never carry, one
of them in Ankara, and wrote orders the adjudicator discarded without
saying why.

What needed nothing added: the order list already flags A Yorkshire → Norway
in red while it stands alone, and clears it when F North Sea convoys
Yorkshire → Norway joins it. The validator knew all along.

README.md

@@ -271,6 +271,37 @@ the order takes you back to the start of it. And the unit you picked to
 support is marked on the board in a different colour from the unit giving
 the order, because otherwise the second half is written blind.
 
+## Ordering a convoy
+
+The rules have had convoys all along -- fifty of the DATC cases are about
+them, including the paradoxes -- but for a while you could not order one.
+The board offered an army the provinces it could walk to, and a crossing is
+by definition not one of those, so the army's half of a convoy could not be
+written. The fleet's half could, which made it worse: the verb was there and
+did nothing anybody could use.
+
+An army on a coast is now also offered the coasts a chain of crewed seas can
+reach, drawn in blue and with long dashes so a crossing does not look like a
+march. A sea counts if there is a fleet standing in it, whoever owns it. A
+fleet that has not been ordered to convoy still counts: it is a thing that
+could be arranged, which is what the negotiation is for, and the adjudicator
+will bounce the crossing if it was not.
+
+That is deliberately not the question the rules ask when they *judge* a
+convoy, which is whether water could ever get there. That one says yes to
+most of Europe. Offering thirty provinces because a chain of fleets is
+conceivable would be worse than offering none.
+
+Clicking a blue coast writes the move `viaConvoy`, which is not a formality:
+the rules allow a unit to be convoyed to a province it could have walked to,
+and the two orders resolve differently.
+
+The two halves are written separately, as they always have been, and the
+order list says so while only one of them exists -- `A Yorkshire → Norway`
+sits there in red until `F North Sea convoys Yorkshire → Norway` joins it,
+and then both go black. Nothing was added to make that happen. The validator
+already knew.
+
 ## Still to build
 
 - bots strong enough to solo against each other, not only to draw