Finish the broker-port migration: _build_broker still constructs CoinbaseClient directly

[eaitbrahim]

Raised in docs/presentations/se.md §9.1, and split out of the #502 design pass as the stage that makes "port-clean" literally true.

The gap

keel has a ports-and-adapters design: keel-broker-api is the port, and four adapters implement it (coinbase, alpaca, robinhood, kraken) plus fake. All are conformance-tested in CI.

None of them is on the live path. keel/commands/_common.py::_build_broker constructs keel/data/cb_client.py::CoinbaseClient directly, and load_broker/discover_brokers — the keel.brokers entry-point registry — are not called anywhere in the application, only in tests.

The consequence is understated in most descriptions of the architecture: the executor does not bypass the port for the bracket, it bypasses it for every order. CoinbaseClient's whole surface is pre-port — preview_order(product_id, side, order_configuration) and place_order(...) take raw dicts, and executor._order_configuration builds market-order dicts the same way _bracket_order_configuration builds the bracket dict.

This is why keel-broker-robinhood is a dev-only dependency rather than a runtime one: nothing constructs it, so shipping an Ed25519 stack (pynacl) to every install would buy nothing.

Known blockers — the port is missing methods the executor needs

Established during the #502 design pass:

1. No product-catalog port method. executor._base_increment_for (executor: SELL base_size is still serialised at full Decimal precision (follow-up to #513) #516) needs base_increment from list_products, which the port does not expose.
2. Balances shape mismatch. _fetch_available_quote probes a dict-shaped get_accounts response; the port returns list[Balance].

Both are small, but they are why the flip is not a one-line change.

What the flip deletes

Worth stating, because it is the payoff:

• executor._bracket_order_configuration's legacy renderer and its parity test (introduced as the bridge in Broker port has no bracket/OCO order kind — live trailing stops, BE-rolls and scale_out are blocked on it #502 stage 1/2)
• the dual-shape probes — ConfirmFn, _preview_book's dict arm, place_result.get("success")
• the capabilities() grandfather clause for the pre-port client
• the reason Robinhood and Alpaca cannot be selected at runtime

Acceptance criteria

• _build_broker resolves the venue through load_broker/discover_brokers rather than constructing a client directly.
• The port exposes what the executor needs (product catalog / increments, and a balances read the executor can use) — or the executor is adapted to the port's existing shapes.
• The live path places every order through OrderSpec values, not raw dicts.
• The legacy renderer, dual-shape probes and grandfather clause are deleted, not left beside the new path.
• A deployment can select a non-Coinbase venue and have it either work or refuse explicitly at arm time.

Notes

• Do not gate safety work on this. The Broker port has no bracket/OCO order kind — live trailing stops, BE-rolls and scale_out are blocked on it #502 design deliberately does not: the roll-protocol hardening (_roll_stop: a crash between cancel and replace leaves the position naked AND silent #519, shipped) and the stop-management primitives work against the pre-port client today, and holding a safety feature hostage to a refactor would be backwards.
• Related: Broker port has no bracket/OCO order kind — live trailing stops, BE-rolls and scale_out are blocked on it #502 (stage 4 of its plan), Venue visibility must be capability-based, not key-presence-based — a read-only key looks identical to a working one #233 (capability-based venue visibility, which is gated on a venue selector existing), executor: SELL base_size is still serialised at full Decimal precision (follow-up to #513) #516 (the increment read the port lacks).