docs(phase-31): record the 8k-vs-16k question as UNRESOLVED, with the A/B that settles it

Drew asked whether waves cracked better before MAXTOK went 8000 -> 16000. Recording both halves
of the answer so next session does not relitigate it from memory:

CLEAN: raising to 16k did cause a real regression — draft completion 84-89% (8k) -> 41% on wave
bt, 69% on bu — but the cause was a harness interaction, not the model. A 16k generation runs
~530 s at ~30 tok/s while STRAGGLER_GRACE was 120 s, so agents were cut off mid-thought with no
draft. Grace at 700 s fixed it; completion has run 97-99% since.

CONFOUNDED: on banks per draft the 8k era looks better (S59: 1,335 of 2,996 = 44.6%; today's best
16k waves dd 34.7%, de 29.7%) — but the populations differ completely. 8k waves had never-drafted
work; today's draw from skeletons that refused six times. Budget and exhaustion moved together, so
neither figure isolates the other. Neither should be cited as evidence about the budget.

AGAINST the simple story: truncated-turn rate is INVERSELY correlated with bank rate (cx 8.7%
trunc/43.9% bank, dd 8.3%/51.4% vs dl 1.3%/0.5%, ej 0.6%/0%). Budget exhaustion driving the
decline would produce the opposite relationship.

THE A/B: split ONE wave's card pool — half the shards at 8k, half at 16k, same generation mix,
same binaries, same gate, same tree, grace 700 s in both arms. Compare banks per DRAFT and per
GATE MINUTE. Holding the population constant is the whole point; every historical comparison
fails exactly there. If 8k matches 16k, the cheaper budget also buys more agents per unit time.

phase-ends/CURRENT_PHASE.md

@@ -164,6 +164,36 @@ writes `docs/tool-designs/frontier-analysis-s60.md` — **READ THAT FIRST NEXT S
 6. **The unwritten law**: 8 cards across 4 waves independently re-derived that the whole-object gate
    needs every sibling matched. Recorded in §283; never written as its own section.
 
+### THE 8k-vs-16k QUESTION — UNRESOLVED, AND THE A/B THAT WOULD SETTLE IT
+Drew asked whether waves cracked better BEFORE the output budget was raised (MAXTOK 8000 -> 16000
+in S59). The record holds one clean measurement and one confound, and they point opposite ways.
+
+**Clean (measured, S59's own ledger).** Raising MAXTOK to 16k DID cause a real regression — overlay
+draft completion fell from **84-89% (8k) to 41% on wave bt and 69% on bu**. The cause was a harness
+interaction, not the model: at ~30 tok/s a 16k generation runs ~530 s while STRAGGLER_GRACE was
+still 120 s, so agents were guillotined mid-thought with NO draft at all. Raising the grace to 700 s
+fixed it; completion has run 97-99% since.
+
+**Confounded (cannot be resolved from existing data).** On BANKS PER DRAFT the 8k era looks better:
+S59 records the default lane at **1,335 banked of 2,996 drafts = 44.6%**, while today's best 16k
+waves ran dd **217/625 = 34.7%** and de **192/647 = 29.7%**. But the two eras drew from different
+populations — 8k waves still had never-drafted work, today's draw from skeletons that have refused
+six times each. Token budget and population exhaustion moved TOGETHER, so neither number isolates
+the other. Do not cite either as evidence about the budget.
+
+**Separately measured today, and it rules out the simple story**: truncated-turn rate is INVERSELY
+correlated with bank rate — the best waves had the MOST truncation (cx 8.7% trunc / 43.9% bank;
+dd 8.3% / 51.4%) and the dead ones the least (dl 1.3% / 0.5%; ej 0.6% / 0%). If budget exhaustion
+were driving the decline that relationship would run the other way.
+
+**THE A/B THAT SETTLES IT (do this next session — it is cheap):** draw ONE card pool and split it
+within a SINGLE wave — half the shards at MAXTOK 8000, half at 16000, everything else identical
+(same generation mix, same binaries, same gate, same tree state). Compare banks per DRAFT and per
+GATE MINUTE, not completion. Holding the population constant is the entire point; every historical
+comparison fails precisely because it does not. Keep STRAGGLER_GRACE at 700 s for both arms, or the
+8k arm wins on an artefact. If 8k matches or beats 16k on banks per draft, the cheaper budget also
+buys more agents per unit time, which compounds.
+
 ### SESSION STOPPED 22:55 — ALL LANES DOWN, TREE CLEAN
 Every campaign process was stopped deliberately at session end (0 alive, verified twice after
 settling). Stop sentinels `.run/ox_campaign.stop` and `.run/auto/STOP` are SET — **delete both