Testing: Remaining scenarios (T-14) #348

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

Scope. S10, S11 (vectors; XFAIL VV2/VV3/VV5/VV6), S13 (failover; on PR 283's builder, promotion XFAIL #285), S14 (limits; after PR 304), S16 (mode matrix: CI runs the binary twice from V2 on; the second run is continue-on-error until V9, then required).

Acceptance. Each scenario green or XFAIL with its issue; S14 shows 20 agents receiving deltas while one connection is refused three ways.