Fix initial value clobbered by resting leading tick

updateCentered wrote the centered tick back to the binding on first
layout — before the deferred scrollTo(value) ran — so the strip's resting
leading tick (range.lowerBound) overwrote the incoming value, then the
scroll targeted the clobbered value. A picker created with value 13 would
snap to 1.

Gate write-backs on a new didCenter flag that flips true only after the
initial scrollTo; split the one-time onAppear guard into didAppear.

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

Author: fl034

Sources/RulerPicker/RulerPicker.swift

@@ -63,6 +63,10 @@ public struct RulerPicker: View {
     /// whose midX is closest to this is the selection.
     @State private var centerX: CGFloat = 0
     /// Set once after first layout so the initial `scrollTo` only runs once.
+    @State private var didAppear = false
+    /// True once the initial `scrollTo(value)` has run; write-backs to `value`
+    /// are ignored before this so the resting leading tick can't clobber the
+    /// incoming value.
     @State private var didCenter = false
     /// True while the selection is actively changing; drives the white flash.
     /// Resets shortly after scrolling settles so the tick fades back to tint.
@@ -148,12 +152,16 @@ public struct RulerPicker: View {
             .compositingGroup()
             .mask { edgeMask }
             .onAppear {
-                guard !didCenter else { return }
-                didCenter = true
+                guard !didAppear else { return }
+                didAppear = true
                 // Defer until the content has laid out, otherwise the initial
                 // scroll is dropped.
                 Task {
                     proxy.scrollTo(value, anchor: .center)
+                    // Only accept selection write-backs after we've centered on
+                    // the incoming value; otherwise the resting leading tick
+                    // clobbers `value` before this scroll runs.
+                    didCenter = true
                 }
             }
         }
@@ -220,7 +228,7 @@ public struct RulerPicker: View {
     }
 
     private func updateCentered(from centers: [Int: CGFloat]) {
-        guard centerX > 0 else { return }
+        guard centerX > 0, didCenter else { return }
         guard let key = Self.nearestTick(centers: centers, to: centerX), key != centered else { return }
         centered = key
         if value != key { value = key }