research: accumulation policy after fees -- no arm beats static DCA (#831)

[eaitbrahim]

Closes #831. Implements the approved design (#834, revision 2).

Three commits, in the order that matters:

1. 7a1e900 feat(sim): keel/sim/accumulation_policy.py. Four allocators; an account with allowance-aware fees and average-cost lots; a joint stationary bootstrap; the D6 decision rule.
   • 47 tests, each seen failing first. 15 of 15 mutants killed; a 16th was equivalent, and its redundant code was removed.
   • Arm A reproduces dca_into_allowlist to the cent under zero fees on gapless candles. It fills at the next open where the benchmark fills at the close, as the design requires, and a second test pins that difference.
2. 4113ed0 The pre-registration. The driver's docstring is the design's §3–§8, frozen. It was pushed at 02:04 UTC before the run. git diff 4113ed0 -- docs/experiments/2026-09-27-accumulation-policy.py is empty: the driver hasn't changed since.
3. bd18cae The record, data and 16 ledger rows (ablation, a_priori, diagnostic_only; chain verified, 114 rows).

Result (headline: 5 years without PAXG, allowance-aware fees, $30,500 deposited)

Arm	Terminal	Sortino	Max DD	P(ΔSortino > 0), 20 / 60-day blocks	Verdict
A static DCA	$46,629	0.523	79.5%	—	baseline
B value averaging	$48,840	0.542	78.5%	0.583 / 0.599	not better
C steered deposits	$47,967	0.564	79.1%	0.365 / 0.407	not better
D selective trimming	$52,919	0.611	79.3%	0.363 / 0.434	not better

No arm is better than static DCA after fees, in either run or fee model.

• Trimming (D) wins the historical path by 13.5% but improves Sortino in only 36–43% of 2,000 resampled paths. That's a path-specific result, and the record declines to call it a win.
• Value averaging (B) is the most consistent arm: 58–61% of paths, and about 2 points less drawdown. It is still far from the 0.95 bar.
• Prediction misses are recorded. D was expected to do best on the with-PAXG run and was instead the weakest alternative there. PAXG's slippage is about 100 bp per leg.

Verification

• Full suite: 6832 passed, 3 skipped.
• mypy and ruff are clean.
• verify_chain on the ledger: no problems.
• The record's computed claims were re-checked against the data. Three were corrected before committing, and a causal story I hadn't tested was removed.

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