keel-broker-alpaca Phase A: adapter package + port conformance suite green

[eaitbrahim]

Per the PRD (Phase A / FR-1–FR-8): scaffold packages/keel-broker-alpaca (workspace-pinned pyproject, py.typed, registered under the keel.brokers entry point), implement the keel-broker-api port against Alpaca's Trading API — orders (market/limit, fractional), preview_order surfacing best bid/ask (feeds the #332 warning and the #350 spread gate unchanged), candle history mapped to Granularity (15Min/1Hour/1Day) with data-tier (IEX vs SIP) declared as a capability, balances/positions with honest T+1 settlement state, and a fee model including regulatory pass-throughs on sells — until the conformance suite passes against it. Zero changes to keel/ core. Coordinate with #233 (capability-based venue visibility) — this adapter should be its first consumer. The advertised dream first contribution: contract documented, suite-verified.

[eaitbrahim]

PRD refinement landed (PR #374) after exploring LEAN's Alpaca adapter and alpaca.markets — deltas that land in this issue's scope (PRD §4):

• FR-3 extended: order-type mapping now names stop-market/stop-limit, fractional and notional sizing (keel's risk→USD→fractional shares maps to Alpaca's notional orders directly), native bracket/OCO via order_class: bracket for keel's exit brackets, and MOO/MOC declared available-but-unused.
• FR-11 (new): rate-limit/backoff semantics, and paper vs live host selection (paper-api vs api) as part of the Venue visibility must be capability-based, not key-presence-based — a read-only key looks identical to a working one #233 capability declaration — a paper key must never be mistaken for a live one.
• FR-9 (new): session & calendar awareness is adapter scope — clock/calendar from the venue, not a locally maintained calendar.
• Reference note: LEAN's capability enumeration is a useful checklist; its cash/margin/PDT simulation modeling is explicitly out of keel's scope.

[eaitbrahim]

Plan change (operator, 2026-08-19): this is no longer parked for the prospective contributor — keel's own delivery flow takes it, starting now. The contributor stays welcome to join anytime; the conformance suite carries the correctness bar either way. Prerequisite hygiene landed first: trademark/originality posture (#379 / PR #380).