A generated FX variance swap breaks its own check constraint

The `acme_demo` seed profile fails on its last step. `generate_trades` books a population of trades and one of them is refused by the database:

[8/8] generate_trades: failed — Booking trade fc77dd4a-ef13-5f4d-b590-101ed75bad75
failed: Repository error: ores_q_34: ERROR:  new row for relation
"ores_trading_fx_variance_swap_instruments_tbl" violates check constraint
"ores_trading_fx_variance_swap_instruments_tbl_check1"

`ores_trading_fx_variance_swap_instruments_tbl_check1` is `CHECK ((end_date > start_date))`. The tenant is left in `bootstrapping` with everything else loaded, and `compass smoke ticks –fresh` — which the gate's `ticks-smoke` check runs — cannot pass while this holds.

The interesting part is not that a generated row was bad but that it should be impossible. The product's own constructor refuses the condition:

if (!(terms.end_date > terms.start_date))
    refuse("fx variance swap", "an end date after its start");

(`projects/ores.trading/api/include/ores.trading.api/domain/fx_instruments.hpp`, `construct_fx_variance_swap`.) The planner that generates the population calls that constructor (`construct_fx_batch`, `trade_population_planner.cpp:1523`), and it computes the two dates one from the other:

const auto start = add_months(scope.as_of_date, -static_cast<int>(slot % 6));
const auto end = add_years(start, static_cast<int>(tenor_years));

So a row that violates the constraint reached the database without passing through the check, or had its dates changed after it. Both are worth finding, and the second is the more likely: the booking path, the batch splitter (`instrument_batch_mapper.cpp`) and the wire protocol each carry the two dates, and a swap anywhere on that road is invisible to a unit test that checks one of them.

The next step is to make the failure name the two dates — the error stops at the constraint, so the reader cannot tell whether the generator drew an inverted pair or something downstream swapped it.

• `projects/ores.synthetic/core/src/service/trade_population_planner.cpp` (`fx_trade_planner::plan`, `construct_fx_batch`).
• `projects/ores.trading/api/include/ores.trading.api/domain/fx_instruments.hpp` (`construct_fx_variance_swap`).
• `projects/ores.trading/core/src/repository/fx_variance_swap_instrument_mapper.cpp`, which copies both dates symmetrically.
• Reproduced 2026-10-31 with `./compass.sh smoke ticks –fresh –leave-running` at `75b8fc7132`.

• Capture: a provisioning run that dies cannot be resumed — this failure is what leaves the tenant in `bootstrapping`.