feat: ceiling-hit markers as top-edge ticks on the rolling plot

Flag each plot bucket with whether the output limiter hit the ceiling
(limiter GR > 0.1 dB), carried through ScopeRing as a per-bucket hit field and
folded into history columns. The editor draws a short red tick at the top of any
hit column (one column wide, so runs merge into segments and a lone hit is a
dot); nothing otherwise. Global marker, shown on every channel tab.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib.rs

@@ -46,11 +46,16 @@ struct Codename206 {
     scope_in: [f32; 4],
     scope_out: [f32; 4],
     scope_gr: [f32; 4],
+    /// Max output-limiter gain reduction seen in the current bucket (for the ceiling-hit marker).
+    scope_hit: f32,
     /// Samples accumulated into the current bucket, and the bucket length (= sample_rate / BUCKET_HZ).
     scope_samples: usize,
     scope_bucket_len: usize,
 }
 
+/// Limiter gain reduction (dB) above which a plot bucket is flagged as hitting the ceiling.
+const CEILING_HIT_GR_DB: f32 = 0.1;
+
 impl Default for Codename206 {
     fn default() -> Self {
         Self {
@@ -64,6 +69,7 @@ impl Default for Codename206 {
             scope_in: [0.0; 4],
             scope_out: [0.0; 4],
             scope_gr: [0.0; 4],
+            scope_hit: 0.0,
             scope_samples: 0,
             scope_bucket_len: 1,
         }
@@ -164,6 +170,7 @@ impl Plugin for Codename206 {
         self.scope_in = [0.0; 4];
         self.scope_out = [0.0; 4];
         self.scope_gr = [0.0; 4];
+        self.scope_hit = 0.0;
         self.scope_samples = 0;
     }
 
@@ -287,14 +294,17 @@ impl Plugin for Codename206 {
                     self.scope_in[ALL] = self.scope_in[ALL].max(in_mono);
                     self.scope_out[ALL] = self.scope_out[ALL].max(out_l.max(out_r));
                     self.scope_gr[ALL] = self.scope_gr[ALL].max(g);
+                    self.scope_hit = self.scope_hit.max(self.limiter.gain_reduction_db());
 
                     // Emit a plot bucket every scope_bucket_len samples (~BUCKET_HZ).
                     self.scope_samples += 1;
                     if self.scope_samples >= self.scope_bucket_len {
-                        self.meters.scope.push(&self.scope_in, &self.scope_out, &self.scope_gr);
+                        let hit = if self.scope_hit > CEILING_HIT_GR_DB { 1.0 } else { 0.0 };
+                        self.meters.scope.push(&self.scope_in, &self.scope_out, &self.scope_gr, hit);
                         self.scope_in = [0.0; meters::NUM_CHANNELS];
                         self.scope_out = [0.0; meters::NUM_CHANNELS];
                         self.scope_gr = [0.0; meters::NUM_CHANNELS];
+                        self.scope_hit = 0.0;
                         self.scope_samples = 0;
                     }
                 }