Challenge 1: Identify the Multi-Document Risk — Possible Solution
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Without a transaction, the order insert and the stock decrement are
two entirely independent writes, each individually atomic on its own
but with NOTHING guaranteeing both happen together.

Concretely, in ACID terms: this is a failure of ATOMICITY across the
two documents. If the order insert succeeds but the application
crashes (or throws, or loses its network connection) before the stock
decrement runs, the result is an order that claims to have been
placed for a product whose stock was never actually reduced — an
order exists for inventory that, as far as the database is concerned,
was never taken.

This also risks a CONSISTENCY failure: the database as a whole is
supposed to represent a valid state where "stock remaining" and
"orders placed" agree with each other. An order with no matching
stock decrement is an inconsistent state that a transaction's
all-or-nothing guarantee exists specifically to prevent — either both
writes land, or neither does, so the two collections can never be
observed disagreeing with each other.