Let each Mastermind player set their own code

A generated code takes away half the game. Whether to repeat a colour,
whether to leave one out, whether to do the thing this opponent will not
expect — those are real decisions, and no random number generator makes
them.

So a game now opens in a SETTING phase: each player chooses the code
their opponent will have to break, one after the other, and the guessing
starts once both are down. They choose in turn rather than at once
because the engine only ever has one seat on the clock, and it costs
nothing — neither can see the other's, so going second is not an
advantage.

The same move carries both: while the codes are being set it is your
code, and after that it is a guess at theirs. Which one it means comes
from the phase rather than from a flag on the move, because the phase is
the thing that actually knows — a flag could disagree with it.

An empty code used to mean one thing and now means two — not chosen yet,
or chosen and hidden from the device this copy was sent to — so the
state carries a flag per player. Whether the other player is ready is
not a secret, and a screen has to be able to say "waiting for them"
rather than sit blank.

The computer sets a code at random and does not try to be clever about
it: a code chosen to beat a particular solver has a pattern in it, and a
pattern is the one thing a code should not have. That single choice is
the only random in the app drawn from a source nobody else can see —
every other one is seeded from the position so a host and a client
agree, but a code seeded from anything on the wire could be worked out
by the player who is meant to be guessing it.

A code set on a guest's device reaches the host as a move and stops
there: moves are intents sent to the host and are never relayed to the
other player, so the only copy that reaches an opponent is redacted.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_019Pr6B8xDDtJJcb6WJMw7Ud

docs/RULES-mastermind.md

@@ -7,8 +7,18 @@ A hidden code of coloured pegs, and a fixed number of guesses to break it.
 The pencil-and-paper game is one person setting a code and another breaking it,
 which makes a poor two-handed game: one player would sit and watch.
 
-Here **both players are set a code and both are breaking one**, a guess each in
-turn. Whoever cracks theirs first wins.
+Here **each player sets the code their opponent will have to break**, and then
+both are breaking one, a guess each in turn. Whoever cracks theirs first wins.
+
+Setting your own code is half of Mastermind, and a generated one takes it away.
+Whether to repeat a colour, whether to leave one out, whether to do the thing
+this particular opponent will not expect — those are real decisions, and no
+random number generator makes them.
+
+The codes are chosen one after the other rather than at the same time, because
+the engine only ever has one player on the clock. It costs nothing: neither can
+see the other's, so going second is not an advantage. Once both are down, the
+guessing starts with whoever set first.
 
 That symmetry is why the game ends at the foot of a round rather than the moment
 somebody gets it. If the player who went first breaks the code on their sixth
@@ -56,6 +66,16 @@ its own**: the code you are trying to break is never on your device in the first
 place, so there is nothing to find by digging through what arrived over the
 network.
 
+A code set on a guest's device reaches the host as a move, and stops there.
+Moves travel one way — they are intents sent to the host, never relayed on to
+the other player — so the only copy that ever reaches an opponent's screen is
+the redacted one.
+
+What is *not* hidden is whether the other player has finished choosing. That is
+not a secret, and a screen has to be able to say "waiting for them" rather than
+sit there blank, so the state carries a plain flag per player alongside the
+code.
+
 A guess is scored by the host, which is the only machine that can score it. A
 device asked to score a guess against a code it was never given refuses rather
 than quietly answering "nothing" — tested, because that failure would look like
@@ -66,8 +86,19 @@ and a code breaker wants to see what they were up against.
 
 ## What the computer does
 
-There is no tree to search — a guess cannot be answered badly, and the opponent
-makes no reply — so the whole game is in choosing what to try next.
+**Setting a code**, the computer takes one at random and does not try to be
+clever: a code chosen to beat a particular solver is a code with a pattern in
+it, and a pattern is the one thing a code should not have.
+
+That single choice is drawn from a source nobody else can see. Every other
+random number in this app is seeded from the position so that a host and a
+client agree on what happened — but a code seeded from anything that travels
+over the wire could be reconstructed by the very player who is supposed to be
+guessing it.
+
+**Breaking one**, there is no tree to search — a guess cannot be answered badly,
+and the opponent makes no reply — so the whole game is in choosing what to try
+next.
 
 - **Casual** guesses anything it has not tried, ruled out or not.
 - **Club** never guesses a code the answers have already eliminated. That one