feat(brokers): report robinhood's own sizing bounds on the preview

[eaitbrahim]

First half of #410. Reported, never enforced — see the boundary section below for why that split is the design and not a shortcut.

The gap

trading_pairs carries min_order_amount, asset_increment and max_order_size, and nothing read them — get_trading_pairs was one of two transport methods the adapter never called. A sub-minimum or off-increment order found out at the venue.

preview_order now reads them, reports what they say through Preview.errors, and puts the bounds themselves in Preview.detail, so the human at the confirm gate sees the number an order was measured against rather than a bare sentence.

The denominations are not the same

This is the part that bites, established live on 2026-08-19 across all 89 pairs:

symbol	price	min_order_amount	as QUOTE	as BASE
BTC-USD	68,367.13	0.1	$0.10	$6,836.71
ETH-USD	2,098.51	0.1	$0.10	$209.85
DOGE-USD	0.072886	0.1	$0.10	$0.01

All 63 pairs that carry the field report the same 0.1. A constant cannot be a base-denominated minimum across six orders of magnitude of unit price — it is a venue-wide $0.10 floor in quote currency.

The obvious implementation, base_size >= min_order_amount, would reject every BTC order under 0.1 BTC (~$6,800), the exit path included — a far worse failure than the missing check this issue exists to add.

max_order_size and asset_increment go the other way, both base: max_order_size varies per asset (20 BTC, 6,500,000 DOGE) and only lands on a comparable notional ceiling ($1.37M, $474k) read that way. The names carry it once pointed out — amount is quote, size is base — which is why _PairRules says so at the top and detail labels each key with its denomination.

The boundary: reported, never enforced

place_order does not call this, and test_place_order_does_not_consult_the_sizing_bounds pins that it does not. Two reasons, and the second decides it:

1. Every order this adapter can place is an exit or a protective leg — entries are MarketIOCByQuote, which this venue cannot express at all. A check that refuses on those paths can strand a position or leave one running without its stop.
2. Nobody has ever watched this venue reject an out-of-bounds order, because no order has ever been placed against it (Robinhood: order-object shapes are still doc-only — one real order is needed to verify them #412). Refusing locally would be a guess, and a guess that refuses an order the venue would have accepted is a new failure this package invented.

The off-increment note says outright that the outcome is unobserved rather than predicting one. Enforcement can be revisited once #412 has seen a real rejection.

Absences stay absences

Three cases that must not collapse into a passing check, one test each:

• a pair with no min_order_amount (26 of 89, The probe only inspects results[0], and it cost us a real field — min_order_amount was removed on a false negative #230) → says the minimum was not checked, detail reads unknown, not 0
• a trading_pairs call that fails or returns nothing → says the bounds were not checked
• an unpriced market order → not accused of being below a minimum; its zero quote_size is an absence, not a number

symbol= is passed so the venue filters. #230 is the standing lesson: the probe that read results[0] off the unfiltered 89-row response got BILL-USD and concluded the venue publishes no minimum at all.

Not cached, deliberately — a remembered bound can go stale, and a preview approving a spend against a stale ceiling asserts something it did not check. Cost is one request per confirm-gate call, which is once per human decision, not a loop.

Test fakes

Six pricing tests asserted errors == () with fakes that answer accounts and estimated_price but not trading_pairs. That is an incomplete venue, not a neutral one, so they now supply it via _pairs(); the tests that genuinely mean "the venue did not answer" leave it out and assert the resulting note.

⚠️ Worth knowing for anyone testing packages/* from a git worktree: the venv's editable installs resolve to the main checkout, so PYTHONPATH has to put the worktree's packages first or you are testing main's adapter. keel/ and scripts/ resolve from cwd and are unaffected. This ran green against the unmodified package before I noticed.

Full suite: 3662 passed, 3 skipped, with the worktree packages on the path. ruff check and mypy clean.

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