pm-grid: fix dropped notes in chords (queue USB-MIDI packets)

The bulk MIDI endpoint holds one 4-byte packet until the host reads it (~once
per USB frame), so calling send_bytes twice for simultaneous lane hits dropped
the 2nd note (WouldBlock, silently ignored). Queue note-ons in a VecDeque and
drain one-per-poll, keeping the rest for the next iteration — chords now play in
full (staggered ~1ms, imperceptible) instead of all-but-one.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

rust/pm-grid/src/main.rs

@@ -19,6 +19,7 @@
 #![no_main]
 
 extern crate alloc;
+use alloc::collections::VecDeque;
 use alloc::string::String;
 use alloc::vec::Vec;
 use embedded_alloc::LlffHeap as Heap;
@@ -781,6 +782,9 @@ fn main() -> ! {
     let (mut nextrep_x, mut nextrep_y) = (0i64, 0i64);
     let mut last_frame_us = 0i64;
     let mut hb_us = 0i64;
+    // pending USB-MIDI packets: the bulk endpoint holds one 4-byte packet at a time, so simultaneous
+    // notes are queued here and drained one-per-poll (otherwise the 2nd note of a chord is dropped).
+    let mut note_q: VecDeque<[u8; 4]> = VecDeque::new();
 
     loop {
         let us = now_us();
@@ -842,10 +846,21 @@ fn main() -> ! {
         px = x;
         py = y;
 
-        // ---- scheduler: advance clocks, send a USB-MIDI note-on per lane hit (ch10) ----
+        // ---- scheduler: advance clocks, queue a USB-MIDI note-on per lane hit (ch10) ----
         app.tick(now_ns, |note, vel| {
-            let _ = midi.send_bytes([0x09, 0x99, note, vel]); // cable 0, note-on, channel 10
+            if note_q.len() < 64 {
+                note_q.push_back([0x09, 0x99, note, vel]); // cable 0, note-on, channel 10
+            }
         });
+        // drain the queue to the endpoint: send until it's busy (WouldBlock), keep the rest for the
+        // next poll. This is why chords play in full instead of dropping all but the first note.
+        while let Some(&pkt) = note_q.front() {
+            if midi.send_bytes(pkt).is_ok() {
+                note_q.pop_front();
+            } else {
+                break;
+            }
+        }
 
         // ---- ticker scroll advance (~120ms) ----
         // (uses the frame clock implicitly; scroll_off wraps mod scroll_total)