Task: Keep stated but default-valued elements in the bond export

A bond future export writes back every element the source stated, including one whose value equals its type's default.

• The bond_future container carries a presence flag for every member the schema states as optional, and the reverse mapper emits on presence rather than on a non-default value.
• A bond future that states SettlementDirty, FairPrice or ExpiryLag with the type's default value writes all three back.
• The corpus cannot measure this: no document in external/ore/examples is a BondFuture trade. The check is a crafted probe, and ores.ore.core.tests covers it.

The fix is in the model, not the mapper. bond_future is generated from projects/ores.trading/modeling/ores.trading.bond_future.org, so the presence flags land there and are regenerated, following the presence pass the schedule container already took. Touches the modeling org, the generated container, the reverse mapper, and the SQL if a new column is needed for a presence flag that storage cannot otherwise infer.

Found while reviewing the bond round-trip branch, with a crafted probe. The source stated:

<BondFutureData>
  <ContractName>OE</ContractName>
  <ContractNotional>100000</ContractNotional>
  <LongShort>Long</LongShort>
  <Currency>EUR</Currency>
  <ContractMonth>2026-06</ContractMonth>
  <FairPrice>0</FairPrice>
  <SettlementDirty>N</SettlementDirty>
  <ExpiryLag>0</ExpiryLag>
</BondFutureData>

and the export kept only the first five elements.

reverse_bond_future guards each of the three with a value test (if (f.expiry_lag ! 0)=, if (f.settlement_dirty), if (f.fair_price ! 0.0)=), so a stated default is indistinguishable from an absent element. external/ore/xsd/instruments.xsd:422 declares SettlementDirty as xs:string with minOccurs"0"=, so the stated value is schema-valid and the loss is ours.

The corpus reads zero on this, because no example document is a BondFuture trade. The bond gate cannot regress on a class the corpus never states.

Manual QA scenarios (scaffolded via compass add test_scenario, run through the QA Validation Runner panel) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.

Abandoned at sprint 25 close. Sprint 26 replaces the codegen approaches tried so far with one method: for each module, sync codegen and clear its drift. Sprint 26 opens new, clean stories for each module. This story is not carried forward.