CB-WP-0025 T03: ADR-0013 -- one world after the game, and difficulty that

does not measure the bot

Seven decisions. Two are not what T03 expected, because the review moved
the ground under both.

D1: strategy fusion DOES NOT APPLY, and that is why the affordable option
is also the honest one. Fusion is a defect of aggregating over
determinizations to choose a move -- the search picking different actions
in states a player cannot distinguish. After the game there is ONE WORLD:
the deal is known, so a search over it yields a line executable in the
only world there is. The survey treated fusion as this pass's central
obstacle; it is an obstacle to a playing engine, which we are not
building. The tool answers "given the deal as it actually was, was there a
line that reached the threshold" and is labelled that way on screen --
never "how you should have played".

D4: difficulty is the WINNABLE FRACTION, not a bot's win rate. C4 killed
the bot rate -- two trivial policies span 0-100% on the same deals, and
improving the bot would make the game "easier" without a rule changing. A
measure that moves when the measurer improves is not measuring the thing.
The solver supplies the alternative: over N deals, in what proportion does
a winning line exist. That is a property of the deal distribution and the
threshold, which is what ground-game tunes and what GROUND-WP-0005 is
blocked on. Ships as a table -- winnable fraction, named reference policy,
skill gap -- never one number, with policy/N/seed-range/K in the number's
name. Stated as a LOWER BOUND, since a K-round search cannot see a line
that needed round 1.

D2: search GroundState. The survey's view-only structural boundary is not
implementable -- a view cannot fold events, and it said so in §6. The
guarantee moves to something checkable: every move in a witness is marked
visible or hidden, computed from project(). A witness reads "you could
have won, but two of these six moves needed a card you had no way to know
was coming" -- more useful than either extreme.

D3: bounded exhaustive over the last K rounds, table as one co-operative
agent, K=2 default. Affordable once C6 corrected the premise: joint
branching over the last two rounds is ~5x10^2 / 1.6x10^5 / 5.7x10^5 at
2/3/4 seats. Wording is normative: "no winning line found in the last K
rounds", never "unwinnable". PIMC and ISMCTS rejected -- they add strategy
fusion to a problem that does not have it.

D5: the harness becomes an instrument before any figure is quoted (C3) --
positive controls, --self-test in make self-tests, a make target, and a
PLURAL policy panel, because the spread between policies is what C4
exposed and hiding it would restore the error.

D6: no new crate and no port. The L declaration was an over-declaration
and is recorded as one. D7: the node cost is disputed 5x between author
and reviewer; T04 benchmarks it with criterion and neither existing figure
may be cited, including by this ADR.

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

workplans/CB-WP-0025-could-we-have-won.md

@@ -235,7 +235,7 @@ this project have now caught a false headline that every gate passed.**
 
 ```task
 id: CB-WP-0025-T03
-status: todo
+status: done
 priority: high
 state_hub_task_id: "055630a1-56e0-4ad4-8796-35f444313371"
 ```
@@ -258,6 +258,38 @@ state_hub_task_id: "055630a1-56e0-4ad4-8796-35f444313371"
   if the ADR concludes it is a mode of an existing one, say so, and the
   over-declaration is a chaos-window data point worth recording.
 
+**Done 2026-08-05.**
+[ADR-0013](../decisions/ADR-0013-could-we-have-won.md), seven decisions.
+**Two of them are not what T03 was written expecting**, because the review
+moved the ground under both.
+
+- **D1 — strategy fusion does not apply, and that is why this is
+  affordable.** Fusion is a defect of *aggregating over determinizations
+  to choose a move*. **After the game there is one world**: the deal is
+  known, so a search over it produces a line executable in the only world
+  there is. The survey treated fusion as this pass's central obstacle; it
+  is an obstacle to a *playing* engine, which we are not building.
+- **D4 — difficulty is the WINNABLE FRACTION, not any bot's win rate.**
+  C4 killed the bot rate: two trivial policies span 0–100% on the same
+  deals, and improving the bot would make the game "easier" without a rule
+  changing. The solver supplies a policy-independent measure — *over N
+  deals, in what proportion does a winning line exist* — which is a
+  property of the deal distribution and the threshold, and is what
+  GROUND-WP-0005 actually needs. **The bot rate never was.**
+
+**D2** searches `GroundState` (the survey's view-only boundary is not
+implementable — a view cannot fold events) and moves the guarantee to a
+checkable per-move `visible`/`hidden` marking computed from `project()`.
+A witness reads *"you could have won, but two of these six moves needed a
+card you had no way to know was coming."* **D3** is bounded exhaustive
+over the last K rounds — measured affordable at 2–4 seats once C6
+corrected the premise — with normative wording: *"no winning line found in
+the last K rounds"*, never *"unwinnable"*. **D5** makes the harness an
+instrument before any figure is quoted (C3). **D6**: no new crate, no
+port — **the L declaration was an over-declaration and is recorded as
+one**. **D7**: the node cost is disputed 5× and T04 must benchmark it;
+neither figure may be cited, including by the ADR.
+
 ## Task: specify
 
 ```task