What the store sells, as a registry of value objects.

An item is registered, never compiled in: a deployment declares its own
catalog at boot and nothing below this namespace knows what a `:pro` is. The
sample catalog exists so a fresh checkout runs before anyone has configured
anything.

The registry is a VALUE the caller holds, created by `store`. Two stores
embedded in one JVM own two catalogs.

Which arm of an experiment a visitor sees. Pure.

Assignment is a HASH of the visitor and the experiment, not a coin flip and
not a row in a table. Three consequences, all of them the reason for the
choice: the same visitor always sees the same arm without anything being
stored, two processes agree without talking to each other, and a test can
assert an assignment instead of stubbing a random.

The experiments themselves come from `tokens.edn` — the same declaration the
stylesheet is generated from — so an arm that exists at runtime is an arm
that shipped its CSS and passed the contrast gate.

What an invoice's own state means. Pure.

An invoice is a promise about an amount for a window. Once the window has
passed the promise is void — but money is not, so this namespace answers what
ARRIVING money means as well as whether the invoice may still be charged.

Turning independent price reports into one locked quote. Pure.

The stored price is the only price truth; an amount in any other currency
exists only as a quote, valid for one invoice and one window. Sources that
are stale, too few, or in disagreement produce no quote at all — a store that
invents a rate when its sources cannot agree is a store that sells at the
wrong price.