service: add ex-2 XML loader and carry order side through matching

Ex-2 documents declare users and their market-maker assignments next
to the events, and the loader has to reject files that are
schema-valid but logically inconsistent, which the v1 loader has no
notion of.

Add XMLLoaderV2, which checks the extension, unmarshals the document,
runs it through XMLValidatorV2 and only then replaces the event and
user repositories, so a rejected file leaves the previous catalog
intact. The validator enforces unique positive event ids, non-empty
names and descriptions, commission within 0..90 with a known type, at
least two distinct options, a positive LMSR b or order-book d with
non-negative initial inventory, unique user names, positive initial
cash, and exactly one existing event per market-maker reference. Let
EventMapperV2 take the resolved market maker's name so it lands on the
domain event.

Give Order a side and the placing user, and Trade the seller, so an
order book can match a buyer against a seller. Keep the previous
constructors, defaulting to a BUY with no user or seller, so existing
LMSR call sites stay unchanged. Route LocalMatchingEngine through the
mechanism's placeOrder with side, user and limit price instead of
buy(), and have cancelOrder take the order to cancel.

Also add an isBlocked query to User and Ledger for accounts that went
negative.

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