Opening the preview faded the main track out at focus-acquire time, but the
first preview clip only becomes audible after resolve/load/decode — so on
the way IN the 340 ms main fade completed into silence before the preview
arrived (a gap the user heard; the exit was already fine).
The main fade is now deferred: focusBridge.requestSilence only arms the
commitment and grants; coordinator.beginMainSilence performs the actual
fade-pause, and the service fires it from afterChange on the opening
session's first audible clip (or ITEM_FAILED). Main-cos-out then overlaps
preview-sin-in — a true cross. resumeMain skips the fade-resume when the
fade never fired (close before audible), and beginMainSilence re-checks the
main is sounding before fading. The focus coordinator's acquisition logic
and tests are untouched.
Verified live: preview audible at 275 ms, main played until then, faded
~343 ms, paused at 618 ms and held frozen; close resumed from the exact
position in 15 ms.