pm-grid: swap X/Y tempo buttons + full-screen strobe on the downbeat

- Swap X/Y: X now tempo-up, Y tempo-down (match the physical Scroll Pack layout).
- On the master lane's step 0 (the '1'), flash the entire 17x7 matrix at full
  brightness for 80ms — a visual downbeat strobe.

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

rust/pm-grid/src/main.rs

@@ -145,6 +145,12 @@ impl<I: I2c> Matrix<I> {
         }
     }
 
+    fn fill(&mut self, v: u8) {
+        for b in self.fb[1..].iter_mut() {
+            *b = v;
+        }
+    }
+
     fn get(&self, x: i32, y: i32) -> u8 {
         if (0..17).contains(&x) && (0..7).contains(&y) {
             self.fb[1 + pixel_addr(x, y)]
@@ -239,6 +245,7 @@ struct App {
     beatflash: u8,
     beatflash_off: i64,
     bpm_flash_off: i64,  // while >0 and active, Grid/Pendulum briefly show the Ticker so nudges are visible
+    full_flash_off: i64, // strobe the WHOLE matrix bright on the downbeat ("the 1")
 }
 
 fn master_bar_ns(track: &track_format::Track, tempo: i64) -> i64 {
@@ -294,6 +301,7 @@ impl App {
             beatflash: 0,
             beatflash_off: 0,
             bpm_flash_off: 0,
+            full_flash_off: 0,
         };
         app.load(0, 0, now_ns);
         app
@@ -405,6 +413,10 @@ impl App {
             while now_ns >= self.next[li] {
                 self.step[li] = (self.step[li] + 1) % steps;
                 if li == 0 {
+                    if self.step[li] == 0 {
+                        // the downbeat — strobe the entire matrix bright
+                        self.full_flash_off = now_ns + 80_000_000;
+                    }
                     self.m_steps += 1;
                     let bar = (self.m_steps - 1) / steps as i64;
                     if bar != self.lastbar {
@@ -458,6 +470,12 @@ fn lvl_bright(lvl: u8) -> u8 {
 
 // ============================== RENDERING ==============================
 fn render<I: I2c>(m: &mut Matrix<I>, app: &App, now_ns: i64) {
+    // downbeat strobe: the whole matrix at full brightness on "the 1"
+    if now_ns < app.full_flash_off {
+        m.fill(255);
+        m.show();
+        return;
+    }
     m.clear();
     // a tempo nudge briefly forces the Ticker (so X/Y is visible from Grid/Pendulum)
     let view = if app.view != View::Ticker && now_ns < app.bpm_flash_off {
@@ -708,24 +726,24 @@ fn main() -> ! {
                 app.next_track(now_ns);
             }
         }
-        // X: tempo down (tap -1, auto-repeat; -5 after 1.5s held)
+        // X: tempo up (tap +1, auto-repeat; +5 after 1.5s held)  [X/Y swapped per hardware layout]
         if x && !px {
             held_x = us;
             nextrep_x = us + 350_000;
-            app.set_bpm(app.tempo - 1, now_ns);
+            app.set_bpm(app.tempo + 1, now_ns);
         } else if x && px && us >= nextrep_x {
             nextrep_x = us + 120_000;
-            let d = if us - held_x > 1_500_000 { -5 } else { -1 };
+            let d = if us - held_x > 1_500_000 { 5 } else { 1 };
             app.set_bpm(app.tempo + d, now_ns);
         }
-        // Y: tempo up
+        // Y: tempo down
         if y && !py {
             held_y = us;
             nextrep_y = us + 350_000;
-            app.set_bpm(app.tempo + 1, now_ns);
+            app.set_bpm(app.tempo - 1, now_ns);
         } else if y && py && us >= nextrep_y {
             nextrep_y = us + 120_000;
-            let d = if us - held_y > 1_500_000 { 5 } else { 1 };
+            let d = if us - held_y > 1_500_000 { -5 } else { -1 };
             app.set_bpm(app.tempo + d, now_ns);
         }
         pa = a;