Build the full-visibility lane layout

The lane strip replaces the placeholder board: window width divides
across the lanes' width units (no horizontal scroll, no minimum width,
degenerate compression accepted), a lane of n units flowing its cards
into n round-robin masonry columns via a measurement-cached Layout.
Width has two deliberately opposite controls, both landing here: the
right-edge drag (ported verbatim from the pathfinder's ColumnResize)
freezes the 1x standard at drag start, snaps between integer widths
with the asymmetric shadow-leads tick and 10pt re-entry, grows the
window one standard width per snap so siblings keep their exact
pixels, and rubber-bands at the screen's visible frame — uncapped
otherwise; the Increase/Decrease Lane Width items (new Board menu,
Cmd-Opt-arrows) are the stepper's keyboard face and re-divide the
existing window width instead, never touching the window. Width
changes write through the new .resize WriteOperation ("Couldn't
resize…" in the banner vocabulary, which grows with the surfaces by
design); malformed width values render as one unit and stay untouched
on disk. 27 new tests port the pathfinder's resize-math suite onto
the uncapped range and pin the write path's fidelity.

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

Kanban/Storage/BoardWriter.swift

@@ -1192,6 +1192,7 @@ public enum WriteOperation: Sendable, Equatable, CustomStringConvertible {
     case restore(title: String?)
     case purge(title: String?)
     case style(title: String?)      // updateIndex on behalf of styling flows (03-board-ui.md)
+    case resize(title: String?)     // a lane's `width` — the edge drag and the stepper alike (03-board-ui.md § Lane)
     case importAttachment(filename: String)
     case listAttachments
     case renumberChildren           // order-maintenance sweep (compaction)
@@ -1216,6 +1217,7 @@ public enum WriteOperation: Sendable, Equatable, CustomStringConvertible {
         case .restore: .restore(title: title)
         case .purge: .purge(title: title)
         case .style: .style(title: title)
+        case .resize: .resize(title: title)
         }
     }
 
@@ -1237,6 +1239,7 @@ public enum WriteOperation: Sendable, Equatable, CustomStringConvertible {
         case let .restore(title): Self.phrase("restore", title)
         case let .purge(title): Self.phrase("purge", title)
         case let .style(title): Self.phrase("style", title)
+        case let .resize(title): Self.phrase("resize", title)
         case let .importAttachment(filename): "import attachment '\(filename)'"
         case .listAttachments: "list attachments"
         case .renumberChildren: "renumber children"