CB-WP-0041 done: ADR-0020 refuses the port, and T02 is why

T02 — all chance derives from one root seed. Three chance points, all
reading it: the setup deck shuffle, the setup Lead draw, and the reshuffle
permutation. The Problems deal is not chance at all. So in extensive-form
terms the tree has a single chance node at the root.

That test was wrong first, and the mutation caught it. It compared state
hashes — and GroundState carries `seed` as a field, so "different seeds
differ" was true by construction. Mutating the shuffle away left it green.
It now compares the dealt configuration, and the same mutation fails it: a
wrong-subject error inside the control written for T02.

The reshuffle is a pure function of (seed, round) because K5 requires
deterministic replay, where a real table reshuffles independently. That is
a modelling restriction, not a defect, and it is now pinned.

T03 — commit/reveal checked in both directions: before Reveal each seat
sees its own selection and no other; after Reveal the information sets
merge, because an encoding that hides forever is not commit/reveal either.

T04 — ADR-0020 refuses the EFG port, and the blocker is T02 rather than
T01, which inverts what the workplan expected. Perfect recall looked like
the risk and is a constraint with a known answer: key on observation
histories. Making chance explicit is the expensive one — the reshuffle
would become a real chance node and break the K5 purity that every
recording, replay bundle and trial-note hash depends on. A port would
trade the property this project is built on for one it has never needed.

Track B's first move is therefore a question, not a build: take "is
exploitability meaningful for a co-operative game with a shared threshold"
to OpenSpiel on a toy model, where answering it costs nothing. D4 states
what being wrong looks like — OpenSpiel settling on a toy what three
rounds of policy sweeps could not — and makes watching for it the next
action.

Taxonomy §4.1 records the EFG correspondence with the test that checks
each row, so a later pass starts from a specification rather than a memory.

Chaos window 4 at three declarations. Window 3's verdict is now two
windows behind and should be evaluated rather than restated.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

workplans/CB-WP-0041-the-extensive-form-foundation.md

@@ -2,7 +2,7 @@
 id: CB-WP-0041
 kind: product
 title: "The extensive-form foundation"
-status: active
+status: done
 state_hub_workstream_id: "dacfa1fe-81c3-4c97-b81e-ec66d95a611c"
 ---
 
@@ -112,7 +112,7 @@ drawn*, which is weaker than "the property holds" and is printed as such.
 
 ```task
 id: CB-WP-0041-T02
-status: todo
+status: done
 priority: high
 state_hub_task_id: "e223bf7d-6e4e-4377-8eac-a63e83a60d35"
 ```
@@ -130,11 +130,52 @@ approximate the distribution by sampling seeds.
 - **say what an explicit chance player would cost** — that is the input to
   T04's decision, and guessing it is how a port gets built on a hope.
 
+**Done 2026-08-08.** Three chance points, all reading the same root seed:
+
+| where | what |
+|---|---|
+| `setup` | shuffles the Solution deck (`ChaChaRng::from_seed(seed)`), hands dealt off the top |
+| `setup` | draws the **Lead** (`rng.draw(seats)`) from the same stream |
+| `draw_solution` | reshuffles the discard when the deck empties, with `ChaChaRng::from_seed(seed ^ round)` |
+
+**The Problems deal is not chance at all** — `edition::deal` is a pure
+function of the vendored CSV, so every game gets the same board.
+
+**So in extensive-form terms the tree has a single chance node at the
+root.** `a_game_is_determined_by_its_seed` pins it.
+
+**And that test was wrong first.** It compared *state hashes*, and
+`GroundState` carries `seed` as a field — so "different seeds differ" was
+true by construction. Mutating the shuffle away left it green. It now
+compares the **dealt configuration** (hands, Lead, deck order), and the
+same mutation fails it. **A wrong-subject error inside T02's own control**,
+caught by the mutation rather than by reading.
+
+**The reshuffle is correlated with the root seed, and a real table's is
+not.** The permutation is a pure function of `(seed, round)` — deliberate,
+so replay never re-derives it (GameKernel K5), and the code says so.
+`the_reshuffle_permutation_is_a_function_of_seed_and_round` pins that it
+moves with **both** inputs.
+
+**The consequence is a modelling one, not a defect**: at a table the
+reshuffle is an independent random event. An EFG built from this engine
+inherits the correlation and models a *restriction* of the game as played.
+
+**The cost of an explicit chance player, stated for T04:**
+- the root node is **not enumerable** — 24 Solution cards give 24!
+  orderings — so any EFG over this must **sample** chance, which is what
+  external-sampling MCCFR does and what our seed sweeps already do by
+  hand;
+- making the reshuffle a genuine chance node **breaks the K5 purity that
+  makes replay deterministic**. That is the real cost and it is a
+  tension, not a line of code: our recordings replay *because* chance is
+  a function of state.
+
 ## Task: state the simultaneity encoding, and check it
 
 ```task
 id: CB-WP-0041-T03
-status: todo
+status: done
 priority: medium
 state_hub_task_id: "c95b32a1-5006-4acd-abc9-537543119344"
 ```
@@ -148,11 +189,18 @@ stops doing that. That property is load-bearing for every claim in §1 of
 the research note and is currently only implied by
 `SelectionView::Hidden`.
 
+**Done 2026-08-08.** `a_pending_selection_is_hidden_until_reveal` checks
+**both halves** at four seats: before Reveal each seat sees its own
+selection and no other, and after Reveal the information sets **merge** —
+because an encoding that hides forever is not commit/reveal either.
+
+Mutation-proven: make selections public and it fails naming the seats.
+
 ## Task: decide whether to build the port at all
 
 ```task
 id: CB-WP-0041-T04
-status: todo
+status: done
 priority: medium
 state_hub_task_id: "3010fb0d-84cc-4fa8-8c23-726ee946cd79"
 ```
@@ -171,6 +219,28 @@ are too expensive and Track B borrows vocabulary rather than machinery.
   port is a tier-L trigger and this workplan is M — it may not smuggle one
   in.
 
+**Done 2026-08-08.**
+[ADR-0020](../decisions/ADR-0020-we-do-not-build-the-port.md) — **and the
+answer is no.**
+
+**The blocker is T02, not T01**, which inverts what the workplan expected.
+Perfect recall looked like the risk and turned out to be a *constraint
+with a known answer*: key on observation histories. **Making chance
+explicit is the expensive one** — the reshuffle would become a real chance
+node, and that breaks the K5 purity every recording, replay bundle and
+trial-note hash in this repo depends on.
+
+> A port would trade the property this project is built on for one it has
+> never needed.
+
+**Track B's first move is a question, not a build**: take
+"is exploitability meaningful for a co-operative game with a shared
+threshold" to OpenSpiel on a toy model, where answering it costs nothing.
+
+**D4 states what being wrong looks like** — OpenSpiel settling, on a toy,
+a question three rounds of policy sweeps could not — and makes watching
+for it the next action rather than a hope.
+
 ## Not in this workplan
 
 - **No EFG export, no OpenSpiel integration, no equilibrium computation.**