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>

decisions/ADR-0020-we-do-not-build-the-port.md

@@ -0,0 +1,106 @@
+# ADR-0020: we do not build an extensive-form port, and here is what to do instead
+
+status: accepted
+date: 2026-08-08
+decided by: agent, under the standing loop authorization
+tier: M (a decision *not* to create a capability port; creating one would
+have been L). chaos d8 = 7 → no override. **Declaration 3 of chaos
+window 4.**
+references: [CB-WP-0041](../workplans/CB-WP-0041-the-extensive-form-foundation.md),
+[CB-RES-0009](../research/CB-RES-0009-extensive-form-is-the-lingua-franca.md),
+[`specs/Positioning.md`](../specs/Positioning.md) §4 Track B,
+[`simulators/openspiel.md`](../simulators/openspiel.md)
+
+## Context
+
+[CB-RES-0009](../research/CB-RES-0009-extensive-form-is-the-lingua-franca.md)
+found the extensive-form game is the interchange format between
+describing a game and analysing it, and that clay-borg already has most
+of one. CB-WP-0041 T01–T03 then asked what is actually true of the
+engine, and the answers changed the decision.
+
+## D1 — the three findings this rests on
+
+**T01. Perfect recall fails on the projection and holds on the history.**
+22 violations in 44,938 sampled decision points when the information set
+is taken to be `project(Viewer::Player(seat))`; none when it is the
+seat's observation history. **`project()` is an observation, not an
+information state** — OpenSpiel's own `ObservationString` /
+`InformationStateString` split, found here by measurement.
+
+**T02. All chance is one root node, and the reshuffle is correlated.**
+Three chance points, all reading the root seed; the Problems deal is not
+chance at all. The reshuffle permutation is a pure function of
+`(seed, round)` **because K5 requires replay to be deterministic**, where
+a real table would reshuffle independently.
+
+**T03. Commit/reveal is the EFG simultaneity encoding**, and now checked
+in both directions rather than assumed.
+
+## D2 — we do not build an EFG export port
+
+**Refused, and the reason is T02, not T01.**
+
+T01 looked like the blocker and is not: it is a *constraint* with a known
+answer — key information sets on observation histories. That is a
+specification, and a cheap one.
+
+**T02 is the blocker.** Making chance explicit means the reshuffle
+becomes a genuine chance node, and **that breaks the purity that makes
+every recording in this repo replay**. Our scenarios, the replay bundles,
+`record.rs`'s round trip, the trial notes' state hashes — all of them rest
+on chance being a function of state (GameKernel K5).
+
+> **A port would trade the property this project is built on for a
+> property it has never needed.** Replay is load-bearing for the evidence
+> discipline; equilibrium computation is, so far, a research interest.
+
+**And the port is not the cheap part anyway.** Enumeration is impossible —
+24 Solution cards give 24! root branches — so any analysis samples chance,
+which is what our seed sweeps already do.
+
+## D3 — what we do instead, in order
+
+1. **Write the mapping down, do not implement it.** The EFG correspondence
+   is now established and tested; it belongs in the taxonomy as a stated
+   relationship, so a later pass starts from a specification rather than a
+   memory.
+2. **When Track B opens, take the question to OpenSpiel, not our kernel.**
+   Express a *small* GROUND-shaped game against its API and ask whether
+   exploitability says anything useful for a co-operative game with a
+   shared threshold. **That is a question about game theory, not about our
+   engine**, and answering it in our engine would cost the replay property
+   to learn something a toy model answers.
+3. **If the answer is yes, revisit this ADR.** The cost in D2 is real but
+   it is not infinite: an EFG export that *samples* chance rather than
+   representing it would preserve K5 and might be enough.
+
+## D4 — what this decision would look like if it were wrong
+
+**If exploitability turns out to be the instrument that settles design
+questions** — if it answers *"does ATTACK ever pay"* in a way three review
+rounds of policy sweeps could not — then refusing the port will look like
+protecting a process property at the cost of the product's purpose.
+
+**The tell would be**: OpenSpiel answering, on a toy model, a question our
+panels have been unable to answer. **Watch for it. That is D3 step 2, and
+it is deliberately the next thing.**
+
+## Consequences
+
+- No EFG port, no OpenSpiel dependency, no equilibrium computation.
+- **The AM-4 dependency budget is untouched**, which is worth stating
+  because adopting OpenSpiel would have been a large, non-Rust dependency.
+- Track B's first move is a **question**, not a build.
+- The three properties T01–T03 established are now regression-tested, so a
+  later port starts from checked ground rather than re-deriving them.
+
+## What was rejected
+
+| rejected | why |
+|---|---|
+| build an EFG export now | breaks K5 replay purity for a research interest (D2) |
+| key information sets on `project()` | measured wrong — 22 perfect-recall violations (T01) |
+| adopt OpenSpiel as an engine dependency | large non-Rust dependency, for a question a toy model answers |
+| declare Track B blocked | it is not blocked, it is *specified*: the constraint is known and the next step is a question |
+| say nothing and revisit later | the three findings would be re-derived from memory, which is how this project produces wrong-subject errors |