A reserved key comes to life — the card window's sidebar grows a Labels section, and labels stops being somebody else's

`labels` has been a reserved tracker key since the rewrite: preserved verbatim, never
interpreted, drawn only as an anonymous row in the Details section beside `assignees` and
`due`. The owner's cards claim it for first-party use, so it joins the schema — read
leniently (a list of names, a bare scalar coercing to one, a mapping malformed and
preserved), written canonically (a quoted flow list in the order the user arranged, no
auto-sort), and removed outright when the last label goes, the way an expanded lane drops
`collapsed`.

Identity is case-insensitive and display is case-preserving, so a card carries `bug` once
however many ways the board spells it, and entries the reading cannot name ride through the
write untouched at the tail.

The section sits second, above Details — which is the point rather than a layout preference:
Details is where keys the app does *not* own are shown, and this key just stopped being one.
Rows rather than chips, because the sidebar is twenty-six characters wide. The add field
autocompletes against the board's own used-labels universe, derived from every live and
trashed card with no store beside the files, and says out loud when Return would mint a word
the board has never used.

Writes ride a `.relabel` operation of their own, because the commit composer has said
"Relabel card 'X'" since long before there was a control to press — and now the undo row says
it too.

Claude-Session: https://claude.ai/code/session_014PtZdPwqZuqEDLc6wZMtEy

Kanban/Storage/BackgroundField.swift

@@ -155,7 +155,12 @@ extension FrontmatterDocument {
     /// preserved subkey's number or timestamp is emitted by the same code that writes `order` and
     /// `created`. Nested collections recurse, which keeps an unknown subkey holding a list from
     /// being flattened into its `description`.
-    private static func flowText(_ value: YAMLValue) -> String {
+    ///
+    /// **Internal rather than private since 2026-08-09**: `labels` writes a flow *sequence*
+    /// (`LabelsField.swift`) and carries preserved non-name entries through it, which is the identical
+    /// question one collection over — a second copy of this switch would be a second opinion about how
+    /// somebody else's YAML is re-emitted.
+    static func flowText(_ value: YAMLValue) -> String {
         switch value {
         case .null: "null"
         case let .bool(value): FrontmatterValue.bool(value).yamlText