Review round 1: the tolerance now requires the accumulated tail to remain a
prefix of the controlled prompt, so a prompt that arrived and was invalidated by
later output keeps the plain silence bound instead of waiting a tolerance with
nothing left to wait for. The field is validated as zero or at least one
`pollIntervalMs`, matching the handoff grace rule, and tests pin the extended
bound's expiry, the invalidated tail, and that rule. The owning readiness design
note records the new field and Tier 2 bound and cross-links this decision.