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>

README.md

@@ -374,6 +374,19 @@ daemon releases everything on `atexit`, so notes cannot hang.
 Preview requests use **non-persisted** `ExtState` — persisting them would write
 to `reaper.ini` on every keypress.
 
+**Preview routing.** Preview notes go out through `StuffMIDIMessage`, i.e.
+REAPER's virtual MIDI keyboard, which is only heard by tracks that are armed
+and monitoring — normally not the track you are editing, so notes would sound
+through whatever unrelated instrument happened to be armed. Grid mode therefore
+borrows the record routing: on entering grid mode, and again whenever the
+editor moves to an item on a different track, the edited take's track is armed
+and monitoring with MIDI input, and every other armed track is stood down so it
+cannot answer as well. The previous arm, monitor and input setting of every
+track touched is remembered and handed straight back when grid mode is switched
+off, when the MIDI editor closes, or when REAPER exits — the daemon does the
+restoring, in the same way it restores the loop and repeat state the loop-bar
+action borrows.
+
 ## Tunables
 
 In `ExtState` section `midigrid`:
@@ -400,9 +413,11 @@ itself.
 
 ## Limitations
 
-- Preview uses `StuffMIDIMessage`, which plays through the track's instrument
-  via the virtual-keyboard input, so the track must be armed / monitored —
-  exactly as with REAPER's own MIDI preview.
+- While grid mode is on, the edited track is held record-armed and monitoring
+  with MIDI input so that preview can reach its instrument (see *Preview
+  routing* above), and any other armed track is stood down. Both are handed
+  back when grid mode ends, but a REAPER crash mid-session would lose that
+  arm state.
 - The grid aligns to the project timeline (bar lines), not the item start. For
   an item starting off-grid, the first cell will be partial.
 - Note octave naming follows the `midioctoffs` preference; with the default,