Compile tests cover decideMode indirectly; these direct tests pin
every branch so a regression in the classifier itself can't slip
behind a more elaborate Compile failure.
Pinned: nil rule and Disabled status both → Disabled; ExpirationDate
→ ScanAtDate without consulting retention; metaLogRetention=0 means
unbounded so any horizon → EventDriven; horizon within retention →
EventDriven; horizon exceeding retention → ScanOnly; bootstrapLookback
adds to horizon (not retention) so a near-threshold case is still
gated; zero horizon (rule field unset) skips the gate. RuleMode.String
must render the documented names for every variant; an unknown value
collapses to "unspecified" rather than empty or panic.
