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/LiveStore/TransientBoardState.swift

@@ -323,6 +323,18 @@ public final class TransientBoardState {
     /// out from under a commit that was about to read its draft.
     public private(set) var renameEditor: RenameEditor?
 
+    /// The open Style… popover's target, or `nil` when no popover is open (03-board-ui.md § Styling
+    /// ▸ Controls). See `StyleEditorSession` for the lifecycle it implements and why it lives here
+    /// rather than in a view.
+    ///
+    /// **Not an inline editor**, deliberately: it is not a title field, it does not hold the text
+    /// domain's keyboard, and `isEditingInline` must stay the answer to "may board commands run" —
+    /// Style… itself is *disabled* while an inline editor is open, so the two never coexist anyway.
+    ///
+    /// `private(set)` for `renameEditor`'s reason: its two transitions are the methods below, and a
+    /// session assignable from anywhere could be re-aimed behind the reload rule's back.
+    public private(set) var styleEditor: StyleEditorSession?
+
     /// Whether a title editor holds focus — 04-interactions.md's **focused-editor rule** as one
     /// boolean: "while an inline title editor — rename or the new-card placeholder — is focused,
     /// board-scoped menu commands (Delete, New Card, Paste, Move, Style, …) disable via menu
@@ -466,6 +478,26 @@ public final class TransientBoardState {
         renameEditor = nil
     }
 
+    // MARK: - The style editor's lifecycle
+
+    /// Opens the Style… popover on `target` — the menu item's call and both context menus'
+    /// (03-board-ui.md § Styling ▸ Controls).
+    ///
+    /// Replacing any session already open, for the inline editors' reason turned inside out: there is
+    /// one popover, so a second Style… is the first one being re-aimed by a fresh gesture. Unlike
+    /// `beginRename`/`beginPlaceholder` it does **not** clear the inline editors — it cannot coexist
+    /// with one (Style… disables while an editor is focused), so clearing them here would be a rule
+    /// about a state that menu validation already rules out.
+    public func beginStyleEditor(for target: StyleTarget) {
+        styleEditor = StyleEditorSession(target: target)
+    }
+
+    /// Closes the popover — the user dismissing it, and the anchor's response to a session the
+    /// reload rule emptied.
+    public func discardStyleEditor() {
+        styleEditor = nil
+    }
+
     // MARK: - Reload
 
     /// The one reload hook: re-grounds every piece of this container on a freshly applied snapshot.
@@ -508,6 +540,13 @@ public final class TransientBoardState {
     /// current universe does not have. It is not an `ItemReferenceSet` only because it is one
     /// optional rather than a set on a side — the rule it obeys is the same one.
     ///
+    /// **The style editor tracks its target set live** (03-board-ui.md § Styling ▸ Controls,
+    /// settled): a member that vanishes or flips liveness leaves the set — so the editor's
+    /// mixed-state display recomputes off the survivors — and a set emptied by a foreign reload
+    /// clears the session, which is how "the popover dismisses when it empties" reaches the screen.
+    /// It never becomes a board session on the way; `StyleEditorSession.resolved(against:)` owns
+    /// both halves.
+    ///
     /// `searchQuery` and `isTrashVisible` are deliberately not mentioned below. Neither references
     /// an item, so no snapshot can invalidate either — the query's *results* change with every
     /// snapshot, which is precisely why the results are not stored here.
@@ -516,6 +555,7 @@ public final class TransientBoardState {
         dragMembers = dragMembers.resolved(against: snapshot)
         pendingCut = pendingCut.resolved(against: snapshot)
         newCardPlaceholder = resolvedPlaceholder(against: snapshot)
+        styleEditor = styleEditor?.resolved(against: snapshot)
 
         // One universe computed once and asked three questions — the rename target's liveness, the
         // last-active lane's, and (via the placeholder above, which asks its own way) the anchor's.