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>

test_lib.lua

@@ -84,4 +84,49 @@ eq(table.concat(G.chordPitches(69, 3), ","), "69,72,76", "A minor triad")
 ext.root, ext.scale = "0", "1"
 eq(G.chordPitches(127, 3), nil, "chord past top of range")
 
+-- Preview routing: the edited track takes over arm/monitor so preview sounds
+-- through its instrument, any other armed track stands down so it cannot
+-- answer as well, and restore puts every one of them back.
+local tracks = {
+  { guid = "{A}", arm = 0, mon = 0, input = 0 },
+  { guid = "{B}", arm = 0, mon = 0, input = 0 },  -- the track being edited
+  { guid = "{C}", arm = 1, mon = 1, input = 6112 }, -- somebody else's armed track
+}
+local FIELD = { I_RECARM = "arm", I_RECMON = "mon", I_RECINPUT = "input" }
+reaper.CountTracks = function() return #tracks end
+reaper.GetTrack = function(_, i) return tracks[i + 1] end
+reaper.GetTrackGUID = function(t) return t.guid end
+reaper.GetMediaTrackInfo_Value = function(t, k) return t[FIELD[k]] end
+reaper.SetMediaTrackInfo_Value = function(t, k, v) t[FIELD[k]] = v end
+reaper.GetMediaItemTake_Track = function(tk) return tk.track end
+
+local take = { track = tracks[2] }
+G.routePreview(take)
+eq(tracks[2].arm, 1, "edited track armed")
+eq(tracks[2].mon, 1, "edited track monitoring")
+eq(tracks[2].input, 6112, "edited track takes MIDI input")
+eq(tracks[3].arm, 0, "other armed track stood down")
+eq(tracks[1].arm, 0, "unarmed bystander untouched")
+
+-- Re-pointing at the same track must not re-save the borrowed state as if it
+-- were the original, or restore would hand back grid mode's own settings.
+G.routePreview(take)
+G.restoreRouting()
+eq(tracks[2].arm, 0, "edited track disarmed again")
+eq(tracks[2].mon, 0, "edited track monitoring restored")
+eq(tracks[2].input, 0, "edited track input restored")
+eq(tracks[3].arm, 1, "other track re-armed")
+eq(tracks[3].mon, 1, "other track monitoring restored")
+
+-- Restoring twice is harmless, and switching to a take on another track
+-- returns the first one before borrowing the second.
+eq(G.restoreRouting(), false, "restore with nothing borrowed")
+G.routePreview(take)
+G.routePreview({ track = tracks[1] })
+eq(tracks[1].arm, 1, "second track armed")
+eq(tracks[2].arm, 0, "first track handed back on switch")
+G.restoreRouting()
+eq(tracks[1].arm, 0, "second track handed back")
+eq(tracks[3].arm, 1, "bystander still armed at the end")
+
 print(fails == 0 and "ALL PASS" or (fails .. " FAILURES"))