Add TestCheckStatus_SampleMemoryBounded asserting that, across 100k ticks,
the per-(alert,server) sample slice length and capacity stay bounded by the
rule's retention window and never grow with elapsed time.
Trim alert samples using AlertRule.RetentionWindow() (max rule duration)
instead of Check()'s verdict max, which is 0 while a rule's window is still
filling. The old max<=0 trim wiped history every tick, so Duration>=2 general
rules never accumulated enough samples and never fired a notification.
Add model + end-to-end singleton regression tests covering sample
accumulation, the retention-window contract, and the actual notify path.
naiba