Fix the collapsed-lanes render on initial board open

The strip's standard-width division read the drop registry's strip
frame during body evaluation, but the registry is deliberately
unobserved (it exists for event-time reads): the first body ran
against a zero frame, every lane collapsed to the 1pt floor, and
nothing invalidated the view when the real measurement landed — the
board stayed corrupted until an unrelated re-render. The layout now
reads the GeometryReader's live proposal (reactive by construction);
the drop delegates keep reading the registry at event time.

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/UI/Board/BoardView.swift

@@ -121,7 +121,7 @@ struct BoardView: View {
     private let spacing: CGFloat = 12
 
     var body: some View {
-        GeometryReader { _ in
+        GeometryReader { proxy in
             // The strip's slots: the lanes the strip should *show*, with the drag's N contiguous
             // shadows opened at the proposal. Recomputed on every render, so a foreign reload
             // mid-drag simply moves the zones (rule 1 of 04-interactions.md ▸ Drag and drop's
@@ -129,7 +129,7 @@ struct BoardView: View {
             let slots = stripSlots
             ZStack(alignment: .topLeading) {
                 backdrop
-                laneStrip(slots)
+                laneStrip(slots, standard: standardWidth(stripWidth: proxy.size.width))
             }
             .padding(spacing)
             .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .topLeading)
@@ -254,8 +254,7 @@ struct BoardView: View {
 
     /// The lanes and the drag's shadows, plus the trash column when it is shown.
     @ViewBuilder
-    private func laneStrip(_ slots: [StripSlot]) -> some View {
-        let standard = standardWidth
+    private func laneStrip(_ slots: [StripSlot], standard: CGFloat) -> some View {
         HStack(alignment: .top, spacing: spacing) {
             ForEach(slots) { slot in
                 switch slot {
@@ -480,11 +479,20 @@ struct BoardView: View {
             gap: spacing,
             window: window,
             stripFrame: { laneDrops.stripFrame },
-            standard: { standardWidth }
+            standard: { standardWidth(stripWidth: laneDrops.stripFrame.width) }
         )
     }
 
-    /// The strip's 1× lane width.
+    /// The strip's 1× lane width, for a strip `stripWidth` points wide.
+    ///
+    /// **The width is a parameter, and which width the caller passes matters.** Body evaluation
+    /// passes the `GeometryReader`'s live proposal, because that is the one source that invalidates
+    /// the view when it changes — the registry's `stripFrame` is deliberately unobserved (it exists
+    /// for event-time reads), so a body computed from it renders the *previous* layout's width: on
+    /// first open that is `.zero`, every lane collapses to `standardWidth`'s 1pt floor, and the
+    /// board stays that way until an unrelated invalidation happens by. The drop delegates pass
+    /// `laneDrops.stripFrame.width` instead, which is the same number read at event time — exactly
+    /// the registry's purpose. The two agree because the padded container fills the reader.
     ///
     /// During a resize session the standard is **frozen** at its drag-start value: the window is
     /// animating mid-resize, so deriving the standard from the live width would feed that animation
@@ -504,12 +512,12 @@ struct BoardView: View {
     /// - a **cross-board lane arrival**'s units while its shadow hovers here, by the same rule read
     ///   from the destination's side: the shadow occupies its units, and the strip has to make room
     ///   for them or the shadow would be drawn at a width the lane will not have.
-    private var standardWidth: CGFloat {
+    private func standardWidth(stripWidth: CGFloat) -> CGFloat {
         if resize.isActive { return resize.standard }
         var units = LaneLayoutMath.totalUnits(of: liveLanes, trashUnits: isTrashVisible ? 1 : 0)
         units += arrivingLaneUnits
         return LaneLayoutMath.standardWidth(
-            stripWidth: laneDrops.stripFrame.width,
+            stripWidth: stripWidth,
             totalUnits: units,
             gap: spacing
         )