worklet: a stop or a new run lets go of stamped keys

[mishan]

A quantised or ahead key's release is a TW_NOTE stamped with a transport
time. When a Stop is stamped between a key's press and its release, the
stop halts the transport at its own time and the release sits in
scheduled_ for a later time the stopped transport never reaches, until
the next load drops it. The synth's voices are silenced by the halt, but
the off is never applied: the key stays in keyRoutes_ and its off is
never recorded, so the keyboard shows it lit. A Play over a running run is
worse: the load and the begin drop the release and beginDue cleared
keyRoutes_ without letting go, so a key on a plain channel keeps
sounding into the new run.

Both stops (stamped and frame-stamped) and every begin now release each
key still in keyRoutes_ the way its on went. KEY_HEARD keys and direct
keys are their player's to let go and are untouched, so a key held
across a Stop on the local keyboard behaves as before.

protocoltest holds free.gen's seat and a plain channel to having nothing
left sounding after a Stop or a Play that falls between a stamped press
and its stamped release.