Story: Hotfix: the trading children write outside the session party

Let the trade booking and trade state eventing tests pass again, so the suite reports on the notification chain rather than on a row its own session cannot see.

• The two eventing tests run under a party-scoped session.
• ores.trading.core.tests passes.

• The model says its test needs a session party, rather than the codegen guessing it. The codegen decides `party_scoped_writes` from the entity's own party_id column, and cannot see that a parent is party-isolated when the parent is a hand-authored table with no modeling org – which is exactly the case for ores_trading_trades_tbl. A model flag, :party_scoped_writes: true, lets the two children that need it say so.
• A party-scoped session does not imply a party column. The template set v.party_id from the session whenever the session was party-scoped. The flag above breaks that implication, so the assignment is now gated on has_party_column as well.

• Whether ores_trading_trades_tbl should be modelled. If it were, the codegen could derive the party isolation of this parent and the flag would not be needed. That is a much larger change.

Done. ores.trading.core.tests passes: 929 assertions in 140 test cases. Only the two generated eventing tests changed, plus the template and the flag that produce them.