feat(broker-api): a product-catalogue read, the first of #524's two blockers

[eaitbrahim]

First of the two blockers #524 names. Touches no live behaviour — nothing calls
get_instrument yet; this gives the executor something to move onto.

The gap

executor._base_increment_for (#516) needs the finest base_size a venue will accept, and gets
it today by calling broker.list_products() and picking through raw dicts for product_id and
base_increment. That is the pre-port CoinbaseClient's shape. The port had no catalogue read
at all, which is why the flip isn't a one-line change.

One product, not the catalogue

get_instrument(product_id) -> Instrument | None. The shape is the caller's own argument:
list_products returns ~900 rows on Coinbase and _base_increment_for caches exactly one per
miss, because Repository.set_state commits per call and caching all of them would mean ~900
fsyncs inside the order-placement path. Coinbase's transport has carried get_product all along.

None is an answer, not a failure — which is why this differs from get_order. An order id
was handed to the caller by this venue, so its absence is a real inconsistency. A product id
comes from an operator's allowlist and may simply not be listed here.

Instrument carries product_id and base_increment and nothing else. Quote granularity and
minimum sizes are the same class of fact and would sit here naturally, but nothing reads them, and
a field no caller reads is a field no test meaningfully checks. __post_init__ refuses a
non-positive increment: it is what a caller quantizes against, so a zero crossing the port is a
division error or a silent zero size on the exit path.

Five adapters, three different truths

adapter	behaviour
coinbase	get_product, unwrapping either envelope; None for missing/unparseable/zero/negative
robinhood	get_trading_pairs(symbol) — the venue's min_order_size is this fact, and the port keeps keel's name for it rather than the venue's
fake	a fixed increment, plus None for a reserved NOT-LISTED id so the conformance suite actually exercises the absent case
alpaca	NotImplementedError
kraken	the stub's standard refusal

Alpaca is the one worth reading twice. Alpaca has /v2/assets/{symbol} carrying exactly this;
this adapter's Transport protocol does not declare it. It raises rather than returning None
deliberately — None would tell the executor a symbol is unlisted when the truth is that nobody
wrote the read, which on the live path means silently skipping quantization for every equity.
Same distinction stage 1 of #502 drew about Alpaca's bracket support: "not written yet", not
"impossible".

Conformance

Two suite tests across all four registered adapters. No capability flag gates them, deliberately:
a product catalogue is not an optional venue feature, it is something every venue has and some
adapters have not been taught to read. NotImplementedError says which of those it is.

Mutation-checked — returning a raw venue dict fails the type assertion; a zero increment fails
before it can be returned at all.

Verification

4292 passed / 3 skipped, ruff clean, mypy clean across 347 files.

What's next on #524

The second blocker: _fetch_available_quote probes a dict-shaped get_accounts response while
the port returns list[Balance]. After that, the flip itself — _build_broker through
load_broker, and the executor placing OrderSpec values instead of raw dicts.

🤖 Generated with Claude Code

https://claude.ai/code/session_01T6yA5khYnJ2qzheArRToQ2