Testing: Scenarios as expected failures (T-03) #337

Part of the test strategy (captain, 2026-10-05: scenarios that test the changes together; cleaner, faster suites; TESTING.md). Epic #305.

Scope. S2 (counter part), S3 (counter part), S4, S5 ("one pass" part), S6, S8 (drop refusal part), S17 (views_at part) written against the target contract and registered as KnownDefect { plan_item: "#315" ... } etc. with Reproduction::Deterministic; the parts that hold today (rows, key isolation, diff on replacement) are required passes in the same scenario.

Acceptance. Each XFAIL names its issue (V1 #308, V9 #315, V13 #316, V11 #318, V7 #314, V14 #320, V10 #317); the suite prints the XFAIL list at the end; an XPASS fails the job (the mechanism exists in testkit/src/known_defect.rs); the issue bodies of V9, V11, V13, V14 gain a line "flips scenario Sn to required".