preecho: back to bottom + sequencer 3-cell height + bigger anchor cells

Three reversibility-correcting tweaks:

- StepSequencer module back to (cols, 3).  Gives the preecho row
  room at the bottom without being clipped by the ScrollArea's
  vertical fill when lanes grow.
- Preecho row moved back BELOW the lane ScrollArea, where it was
  originally.  Pinning it above was my previous attempt at avoiding
  the clipping; the root issue was module height, not stacking order.
- Anchor strip cells 10×14 → 15×21 px (~1.5×).  Click targets
  actually read as toggle boxes now rather than slivers.

489 tests still passing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Tok

src/state/rack.rs

@@ -186,7 +186,7 @@ impl ModuleKind {
     pub fn grid_size(self, grid_cols: u8) -> (u8, u8) {
         match self {
             //                                     W     H
-            Self::StepSequencer => (grid_cols, 2),
+            Self::StepSequencer => (grid_cols, 3),
             Self::LlmConsole => (grid_cols, 1),
             Self::MasterOutput => (grid_cols, 1),
             Self::AcidBass => (4, 7),

src/ui/panels/sequencer.rs

@@ -485,11 +485,6 @@ pub fn draw_sequencer(app: &mut ImpulseApp, ui: &mut egui::Ui) {
 
     let sub_rows = sub_rows_for(seq_steps);
 
-    // Pre-echo row BEFORE the lane scroll area so it always has a
-    // stable position and doesn't get pushed out when the ScrollArea
-    // expands to fill the remaining space.
-    super::sequencer_preecho::draw_preecho_row(app, ui);
-
     egui::ScrollArea::vertical().show(ui, |ui| {
         // Prefix width computed statically from the fixed 5-widget prefix
         // structure (M + S + label + vol + steps, separated by item_spacing).
@@ -940,7 +935,9 @@ pub fn draw_sequencer(app: &mut ImpulseApp, ui: &mut egui::Ui) {
         );
     });
 
-    // Pre-echo row is rendered above the lane ScrollArea (see earlier in
-    // this function) so it stays pinned to a stable position instead of
-    // getting pushed past the module bounds when the scroll area grows.
+    // Pre-echo row lives at the bottom of the panel below the lane
+    // ScrollArea.  The module is sized (cols, 3) specifically so this
+    // row has room to sit without being clipped by the scroll area's
+    // greedy vertical fill.
+    super::sequencer_preecho::draw_preecho_row(app, ui);
 }

src/ui/panels/sequencer_preecho.rs

@@ -16,8 +16,10 @@ use crate::sequencer::PreechoConfig;
 use crate::ui::{ImpulseApp, theme, widgets};
 
 /// Width of each step cell in the anchor strip.
-const ANCHOR_STEP_W: f32 = 10.0;
-const ANCHOR_STEP_H: f32 = 14.0;
+/// Sized ~1.5× the original 10/14 so the click targets read more
+/// like toggle boxes than slivers.
+const ANCHOR_STEP_W: f32 = 15.0;
+const ANCHOR_STEP_H: f32 = 21.0;
 
 const VOICE_KEYS: &[&str] = &["kit_a", "kit_b", "amen", "bass", "hoover", "an1x"];