Build the styling system and shared style editor

One style-editor component, anchor-agnostic: a background grid (None
well plus the 12 palette colors) and a curated symbol grid (the
pathfinder's five-dozen set, leading well removing the icon key for
the level default), selection-aware across cards, lanes, and the
board itself. Batch edits compute per-dimension state — uniform,
mixed (no well selected), or an off-palette value labeled verbatim
outside the grids — and choosing a well applies to the whole target
set as one write bracket, skipping no-ops per field. The popover
tracks its target set live per the freshly ratified rule: targets
re-resolve by UUID on every reload, a vanished target leaves the set,
an emptied set dismisses the editor, and nothing ever silently
retargets to the board. Anchors landing now: Board > Style
(Opt-Cmd-S) and the card/lane context menus, which also carry the
quick-style recents row (app-wide, persisted, capped at six, None
never recorded) and the lane's width control twinning the menu
chords. The styling system's other two renders arrive with it: a
lane's background paints the C7 top-edge band, the board's paints
the window content background — malformed values paint nothing and
stay byte-identical on disk. 31 new tests.

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

Kanban/UI/Board/BoardView.swift

@@ -45,6 +45,10 @@ struct BoardView: View {
     /// needs the board's own window ref, which is the host's identity and not the board's.
     let openCard: (ItemID) -> Void
 
+    /// The app-wide quick-style recents, for the board-anchored style editor (03-board-ui.md §
+    /// Styling ▸ Controls).
+    @Environment(AppModel.self) private var appModel
+
     /// One resize at a time, per window. `@State` so it lives exactly as long as this board window's
     /// view does, which is the interaction's whole lifetime.
     @State private var resize = LaneResizeSession()
@@ -89,6 +93,12 @@ struct BoardView: View {
             .padding(spacing)
             .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .topLeading)
         }
+        .background(boardBackground)
+        // The board's own anchor for the Style… popover — the surface a board-targeted session hangs
+        // off, since the board has no item to attach to (`styleEditorPresentation`'s `nil` anchor).
+        .popover(isPresented: styleEditorPresentation(store, anchor: nil), arrowEdge: .top) {
+            StyleEditorPopover(store: store, recents: appModel.styleRecents)
+        }
         // The board is a focus target so the grammar keys reach it at all. The focus *ring* is off:
         // the strip is the window's content, not a control, and a rectangle around the whole board
         // would read as an error state.
@@ -105,6 +115,27 @@ struct BoardView: View {
         .onKeyPress(.escape) { handleEscape() }
     }
 
+    // MARK: - Styling
+
+    /// The board's `background`, painting "the board window's content background (the surface behind
+    /// and between lanes)" (03-board-ui.md § Styling ▸ Capabilities).
+    ///
+    /// Unlike the lane band and the card stripe this one is a **fill**, because at board level that
+    /// is what the design asks for — and it is why the board is the level 10-accessibility.md binds
+    /// its ≥ 4.5:1 rule to: text does sit on it. That runtime contrast computation (a hex background's
+    /// text colour, recomputed against the composited backdrop on appearance change) is not this
+    /// card's — what ships here is the palette path, whose twelve pairs are AA-verified at design
+    /// time.
+    ///
+    /// A value that resolves to nothing paints nothing, so the window keeps the standard background:
+    /// the same lenient degrade as the other two levels, and the bytes stay as written.
+    @ViewBuilder
+    private var boardBackground: some View {
+        if let color = Palette.color(for: store.snapshot.background) {
+            color
+        }
+    }
+
     // MARK: - Lanes
 
     /// One lane's strip slot, plus its trailing grab strip.