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/BoardCommands.swift

@@ -121,6 +121,48 @@ struct BoardRenameCommand: View {
     }
 }
 
+// MARK: - Style
+
+/// Board ▸ Style… (⌥⌘S) — the style editor's menu-bar anchor (11-command-nexus.md;
+/// 03-board-ui.md § Styling ▸ Controls).
+///
+/// **Selection-aware, with the board as the empty-selection case**: "Board window: selected cards or
+/// lane; nothing selected = the board". The item does not present anything itself — it opens the
+/// session (`TransientBoardState.beginStyleEditor`) and the board window's anchors decide which
+/// surface hosts the popover, which is what keeps the presentation attached to what is being styled
+/// rather than to the menu bar.
+///
+/// Validation is `acceptsBoardMutations` — the lock and the focused-editor rule, the latter naming
+/// Style in its own list of board-scoped commands (04-interactions.md ▸ Grammar) — plus one rule of
+/// its own: **a tombstoned selection disables it rather than falling through to the board.**
+/// Everything edit-shaped is disabled on tombstoned selections (04 ▸ The trash), and quietly
+/// restyling the board because the user had a trashed card selected would be the silent retarget
+/// 03 forbids.
+struct BoardStyleCommand: View {
+
+    @FocusedValue(\.boardStore) private var store
+
+    var body: some View {
+        Button("Style…") {
+            guard let store, let target = styleTarget else { return }
+            store.transient.beginStyleEditor(for: target)
+        }
+        .keyboardShortcut("s", modifiers: [.option, .command])
+        .disabled(styleTarget == nil)
+    }
+
+    private var styleTarget: StyleTarget? {
+        guard let store, store.acceptsBoardMutations else { return nil }
+        let selection = store.selection
+        guard !selection.isEmpty else { return .board }
+        guard selection.liveness == .live else { return nil }
+        // Re-resolved against the snapshot on the way in, so the session starts out holding only
+        // items that render — the same universe its own reload rule will hold it to.
+        let live = selection.resolved(against: store.snapshot).ids
+        return live.isEmpty ? nil : .items(live)
+    }
+}
+
 // MARK: - Lane width items
 
 /// Increase / Decrease Lane Width — **the width stepper's keyboard face** (03-board-ui.md § Lane,