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/StyleRecents.swift

@@ -0,0 +1,82 @@
+import Foundation
+import Observation
+import os
+
+/// The quick-style row's memory: the backgrounds this user has applied lately, most-recent-first
+/// (03-board-ui.md § Styling ▸ Controls — "Quick-style row, recents only … Recents are app-wide and
+/// persist app-side (user preference, never board data)").
+///
+/// ### Why it lives here, and not in the registry
+///
+/// `BoardRegistry`'s own doc names this type's contents as the example of what does *not* belong in
+/// it: "App-wide state that is not board-scoped — quick-style recents, the SSH host-key table, the
+/// last-used card-window size — lives *beside* this file, not in it" (02-architecture.md §
+/// Per-board app state). No board owns the list; styling one board's cards blue is what makes blue
+/// offer itself on the next board, which is the whole point of the row.
+///
+/// ### Two halves, so the rules are testable without a defaults domain
+///
+/// The list *rule* is `updated(_:with:cap:)` — a static pure function — and this object is its
+/// persistence and its observability. `@Observable` so every open context menu reorders the instant a
+/// colour is applied from anywhere; `defaults` injectable so a test drives a suite of its own rather
+/// than the user's.
+///
+/// ### What never enters the list
+///
+/// **The None well is not a colour.** It removes the `background` key, so there is nothing to
+/// remember and nothing a future row could offer — the recording call site simply never fires for it
+/// (see `StyleCommand.apply`). Empty strings are refused here too, belt over braces.
+@MainActor
+@Observable
+public final class StyleRecents {
+
+    /// How many the row keeps. Six is "capped ~6" from the milestone's shape and about what a
+    /// compact menu row carries without becoming a second palette — which is the one thing the
+    /// design says this row is not ("the pathfinder's second full-palette tier is gone").
+    public static let cap = 6
+
+    /// The recents, most-recent-first. Palette names or `#RRGGBB[AA]` hex — the same strings that go
+    /// to frontmatter, so a hand-written hex applied from a board's file could join the row without
+    /// any translation (nothing in-app can produce one today; § Controls' curated-in-app rule).
+    public private(set) var backgrounds: [String]
+
+    @ObservationIgnored
+    private let defaults: UserDefaults
+
+    private static let logger = Logger(subsystem: "dev.rzen.indie.Kanban", category: "style-recents")
+
+    /// - Parameter defaults: the domain to persist in. Injected for the reason `BoardRegistry` takes
+    ///   its storage URL: a test must be able to hold its own without touching the user's.
+    public init(defaults: UserDefaults = .standard) {
+        self.defaults = defaults
+        // Anything but an array of strings is treated as an empty list rather than as an error: this
+        // is a convenience, and a hand-edited or truncated preference must never be the reason a
+        // context menu cannot open (`BoardRegistry`'s "it never takes the app down", one scale down).
+        backgrounds = (defaults.array(forKey: AppPreferences.quickStyleBackgroundsKey) as? [String]) ?? []
+    }
+
+    /// Records `value` as the most recently used background and persists the new list.
+    ///
+    /// Move-to-front, deduped, capped — see `updated(_:with:cap:)` for the rule itself. Called on
+    /// **every** background application from every anchor (the editor's wells, the quick-style row),
+    /// which is what "recently used" has to mean for the row to be worth having; the write is
+    /// unconditional even when the value was already at the front, since re-persisting an identical
+    /// list is cheaper than deciding not to.
+    public func record(_ value: String) {
+        backgrounds = Self.updated(backgrounds, with: value)
+        defaults.set(backgrounds, forKey: AppPreferences.quickStyleBackgroundsKey)
+    }
+
+    /// The list rule, as a pure function: `value` to the front, its earlier occurrence removed, the
+    /// tail truncated to `cap`.
+    ///
+    /// An empty `value` returns the list unchanged. It is not a colour anyone applied — the two ways
+    /// to arrive at one would be a removal (which is the None well, and never recorded) or a
+    /// hand-written blank — and a blank swatch in the row would be an unclickable hole.
+    public static func updated(_ list: [String], with value: String, cap: Int = cap) -> [String] {
+        guard !value.isEmpty else { return list }
+        var updated = list.filter { $0 != value }
+        updated.insert(value, at: 0)
+        return Array(updated.prefix(cap))
+    }
+}