Preserve final-bar sell signal until next market open

[andywkff]

Problem

When an exit rule fires on the final input bar, the core returns .exit but also synthesizes a completed trade at that bar's close. This contradicts next-market-open execution semantics.

Acceptance criteria

• Final-bar exit remains pending and emits no completed trade for that position.
• Adding the next bar completes the trade at the next bar's open.
• Existing open-position finalization behavior without a pending exit remains unchanged.
• Targeted and full Swift tests pass.

[andywkff]

Fixed on codex/preserve-pending-exit.

• A final-bar exit now remains .exit with no completed trade for that position.
• Supplying the next bar completes the trade at that bar's open with exit-rule.
• Focused regression test passed.
• Full stable Swift suite passed: 128 tests, 0 failures.