Play preview through the edited track's instrument

StuffMIDIMessage feeds the virtual MIDI keyboard, which only reaches
tracks that are armed and monitoring -- normally not the track whose
item you are editing. Opening an item in grid mode therefore sounded
every preview through whatever instrument happened to be armed, which
is confusing when the notes you are writing come back in the wrong
voice.

Grid mode now borrows the record routing: the edited take's track is
armed and monitoring with all MIDI inputs (anything narrower can
exclude the virtual keyboard), and every other armed track is stood
down so it cannot answer as well. The prior arm, monitor and input of
each track touched is saved by GUID in transient ExtState.

routePreview re-points lazily, so moving the editor to an item on
another track costs nothing until the target actually changes. The
daemon hands the routing back when grid mode goes off, when the editor
closes, or on exit -- the same way it already restores the loop and
repeat state the loop-bar action borrows -- so the project is put back
even if grid mode is never toggled off.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

MidiGrid_OpenInGrid.lua

@@ -22,6 +22,9 @@ if not hwnd then G.say("Could not open MIDI editor") return end
 
 G.set("active", 1)
 
+-- Preview must sound through this track's instrument, whatever else is armed.
+G.routePreview(reaper.MIDIEditor_GetTake(hwnd) or take)
+
 -- Start at the item's beginning so the first cell is the item's first cell.
 local pos = reaper.GetMediaItemInfo_Value(item, "D_POSITION")
 reaper.SetEditCurPos(pos, true, false)