docs(experiments): assess US equities and refuse on screening capability

[eaitbrahim]

Asks whether keel can add US stocks — from Coinbase, and if not, from another broker. The answer is no, and the binding gate is keel's own screening capability rather than any venue. That makes stocks "not yet" where cTrader's CFDs (#203) were "never".

Feasibility only. No code changed, no adapter added, no dependency introduced.

Two refusals, each sufficient on its own

1. §71.6 rejects equity-representing instruments BY CAPABILITY. They require share-style business and financial screening keel performs none of. Broker-independent — no choice of venue fixes it and no probe can move it. asset_attestations cannot even express the answer: (asset, sector, backing ∈ {ayn,dayn,native}, pays_yield, source) has no vocabulary for debt ÷ market cap.

2. Coinbase's equity surface has no market data at any price. A live re-run of the committed probe on 2026-08-09 (POST-guarded, create_order never called) found ONE_DAY candles n=0 across 21 trading days, empty market_open_price/market_close_price on regular sessions, and empty price/best_bid_price/mid_market_price on all 1000 products sampled. Order book and market trades 500. Preview 403s.

The venue premise collapsed and the verdict didn't move

Coinbase does expose an equities order path — equity_order_metadata on create-order, quoted from their reference. The brief for this document said otherwise and was wrong. Had the charter gate been a formality, that correction would have flipped the answer. It changes nothing, which is what makes §2 rather than §1 the finding worth keeping.

Corrections against prior work

• The 2026-08-05 study, on two counts: "order path refused by design" rested solely on a preview 403 and was never established; and the non-deterministic-universe finding no longer reproduces (stable cursor walk to 19,188 ids).
• KB §65.10, whose DJIM description is faithful to Ayub but no longer describes DJIM after its 2023 screen retirements — a screen built against it would implement a retired methodology.
• Robinhood: shipped official agentic equities trading 2026-05-27, so "crypto-only" is now false. But it is an MCP server requiring interactive OAuth PKCE consent with no client_credentials grant — unusable from keel's headless launchd runner, and its single internal scope forecloses a data-only integration.
• Schwab: the widely-repeated "AI-driven functionality" scrutiny is not Schwab's language; it traces to a non-affiliated site created 2026-04-20.

AAOIFI SS 21 recorded as decided

Criteria quoted from the standard: debt ≤30% of market cap, interest-taking deposits ≤30% (not cash+securities as DJIM uses), prohibited income ≤5% of total income, tangible assets ≥30%. No receivables test, no averaging window. Two tensions left deliberately open: SS 21 reads as exemption from prohibition rather than permissive tolerance, and clause 3/4/6/1 would exempt keel's ~24-day holds from purification by timing — satisfying the obligation by not being there on the measurement date.

Reviewer note

This document went through three research passes; the first two produced confident, well-formed claims that direct verification reduced, including one fabricated enum that survived inside the very section announcing its withdrawal. The audit trail and a short research-integrity section are in the document deliberately. Unverified claims are labelled as such — most importantly, create_order on an equity has never been called, and §71.6 applied to real US shares remains an inference.

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