Katan: open trade offers and counter-offers

- Offers can go to the whole table (asked in seat order, empty hands skipped) or one seat.
- The asked seat may accept (trade happens at once), decline, or counter with new terms.
- If nobody accepts and there are counters, the offerer takes one or turns them all down (new TRADE_COUNTERS phase).
- AI: fixes an inverted accept test (it weighed what it gives against what it gets backwards), picks the best counter.
- UI: Everyone option in the offer dialog, Counter… in the reply dialog, a counter-choice dialog.
- Tests and docs updated.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_018njn2eMPAYqAoE49hGRg2X

docs/RULES-catan.md

@@ -122,17 +122,32 @@ is the same code path.
 
 ### Domestic
 
-An offer names **one** addressee and two card multisets: what the offerer gives,
-what they want. There are no counter-offers and no open auctions in v1.
+An offer is addressed to **one** seat or, with `to = null`, to **everyone**, and
+names two card multisets: what the offerer gives, what they want.
+
+Seats are asked **one at a time**, in seat order after the offerer (seats holding
+no cards are skipped; an addressed offer asks only its addressee). The seat being
+asked may:
+
+- **accept** — the trade happens at once and the offer closes (first accept wins);
+- **decline** — the offer moves on to the next seat;
+- **counter** (`CounterTrade`) — propose different terms: what they would give and
+  what they want from the offerer. A counter must differ from the offer as made
+  (that is an accept), and both sides must hold the cards. The offer moves on.
+
+When every seat has answered and nobody accepted: with no counters the offer
+simply lapses; with counters the phase becomes `TRADE_COUNTERS` and the offerer
+either **takes one** (`AcceptCounter(seat)`, still checked for affordability) or
+**turns them all down** (`DismissCounters`).
 
 An offer is legal only when the offerer's turn is in PLAYING, the addressee is
 another seat, both legs are non-empty, the two legs are not the same multiset
 (`countOf(give) != countOf(get)` — like-for-like is not a trade), and the
 offerer actually holds the cards being offered.
 
-The addressee may **accept** only if they hold what was asked and the offerer
-still holds what was promised; otherwise only **decline** is offered. A decline
-clears the table and leaves every card where it was.
+A seat may **accept** only if it holds what was asked and the offerer still holds
+what was promised; otherwise only decline (or counter) is offered. A refusal
+leaves every card where it was.
 
 Offers are free-form intents rather than enumerated moves (the space of card
 multisets is combinatorial), so they are validated structurally by
@@ -292,15 +307,12 @@ timing rules; Longest Road and Largest Army worth five; victory at ten.
 - **Year of Plenty two-of-a-kind needs the bank to hold both cards**, not one.
 - **The robber must move.** Some printings allow staying put; this engine always
   requires a different hex.
-- **Offers are one-to-one, no auctions or counter-offers** in v1.
+- **Offers may be open to the whole table** and can be countered; the first accept wins (see Trading).
 
 ## Known gaps
 
-- **No domestic auction or counter-offer.** An offer is addressed to a single
-  seat. Open trading to the whole table, and counter-offers, are the obvious next
-  step and are not implemented.
-- **No player-to-player trade over the wire beyond accept/decline.** The engine
-  supports it; whether the UI surfaces it is the screen task's concern.
+- **No timer or cancel on offers.** An offer waits for each asked seat in turn; the offerer cannot withdraw it mid-round, and a player who never answers stalls the table.
+- **The AI never counters.** It accepts, declines, and picks among counters it is sent, but does not propose them.
 - **No trading away a development card.** Not part of the base game.
 - **The screen is built, not yet verified on two devices.** The Katan screen and
   setup options exist (see [UI-catan.md](UI-catan.md)); the catalogue entry is a