StreamSwitchSmoothSilence

Park the SSS in a silent state and clear its internal PTS / audio- transition bookkeeping, so the next switchSource is treated as a cold-start switch rather than as a continuation of the previous activeSource’s timeline. Used when the SSS is parked between downstream consumer windows (e.g. an HSS routing to a sibling switch) and would otherwise contaminate its later anchored handoff with stale clock state.