Adds #[ignore]d integration test that loads a CheckpointV2 file produced
by alpha_train and verifies CfcTrunk::load_checkpoint accepts it.
Activated via FOXHUNT_SMOKE_CKPT env var on a CUDA-capable host.
Also adds a compile-time witness test confirming Summary.max_drawdown_pct
field exists post-X16 (required by emit_deployability_verdict).
Note: full BacktestHarness end-to-end smoke (running one cell against
fixture MBP-10) requires the v2 forward path to live on the trunk
(X11 — deferred). This commit verifies the producer/consumer wire-up.
Per spec §3.4, §4.1 (X18).