F1 cache-analyzer regression guard (SC5): pin percentile (wall_time_ms_p50/p90)
and time-range (oldest/newest_event_iso) — 2 of 3 'unchanged' categories were
previously un-asserted on the mixed-input fixture.
F2 lastMainChainModel: add direct coverage — last-wins across 2 distinct
main-chain models, sidechain exclusion (even when the sidechain is the last
record), and model-absent → null propagating to deriveCost refuse-to-estimate.
804 -> 807 tests (805 pass / 0 fail / 2 skipped). No test pins the test count.
