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/ItemSymbol.swift

@@ -32,6 +32,18 @@ enum ItemSymbol {
         return name
     }
 
+    /// The default for a level — the style editor's symbol grid needs the three defaults as a
+    /// function rather than as three constants, because its leading well is "the level's default
+    /// symbol" and the editor is one component serving all three (03-board-ui.md § Styling ▸
+    /// Controls).
+    static func `default`(for level: StyleLevel) -> String {
+        switch level {
+        case .board: board
+        case .lane: lane
+        case .card: card
+        }
+    }
+
     /// Whether the running system can draw `name` as an SF Symbol.
     ///
     /// Uncached deliberately. The lookup is a bundle-backed symbol resolution that AppKit itself