Didactic hover explanation for the current validation mode. Mode-
specific so the tooltip always explains what the shown value means
(the generic "could be any of three modes" wording read as evasive).
Shared by the initial template and the cache-tab incremental updater
so the tooltip stays correct when the mode changes after the first
validation completes (e.g. '—' → Content Hash).
Didactic hover explanation for the current validation mode. Mode- specific so the tooltip always explains what the shown value means (the generic "could be any of three modes" wording read as evasive). Shared by the initial template and the cache-tab incremental updater so the tooltip stays correct when the mode changes after the first validation completes (e.g. '—' → Content Hash).