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

KanbanTests/FixtureBoardTests.swift

@@ -158,7 +158,10 @@ struct FixtureRichBoardTests {
         // schema-owned keys (schema, title, created, modified, modified-by, background, icon,
         // iconColor) are filtered out; only the agent-overlay and reserved keys remain, in the
         // order they were written.
-        #expect(result.model.document.unknownFields.map(\.key) == ["project", "sphere", "labels", "template"])
+        #expect(result.model.document.unknownFields.map(\.key) == ["project", "sphere", "template"])
+        // `labels` is schema-owned since 2026-08-09 (`FrontmatterKeys.labels`) — filtered out of the
+        // list above at every level, and preserved in the document exactly as it always was.
+        #expect(result.model.document.rawValue(for: FrontmatterKeys.labels) != nil)
     }
 
     /// The whole-tree round-trip guarantee (01-storage-format.md § Fractal layout: "the app
@@ -324,13 +327,16 @@ struct FixtureUnknownKeyOrderTests {
     @Test func unknownKeysPreserveDocumentOrderAtEveryLevel() throws {
         let result = try loadFixture("Valid/unknown-key-order.kanban")
         let model = result.model
-        #expect(model.document.unknownFields.map(\.key) == ["project", "sphere", "template", "labels", "custom-note"])
+        #expect(model.document.unknownFields.map(\.key) == ["project", "sphere", "template", "custom-note"])
 
         let lane = try #require(model.lanes.first)
         #expect(lane.document.unknownFields.map(\.key) == ["remote-state", "assignees", "due", "custom"])
 
         let card = try #require(lane.cards.first)
-        #expect(card.document.unknownFields.map(\.key) == ["labels", "assignees", "due", "remote", "agent-scratch"])
+        #expect(card.document.unknownFields.map(\.key) == ["assignees", "due", "remote", "agent-scratch"])
+        // `labels` is schema-owned since 2026-08-09 — filtered from the list, read as a field, and
+        // still sitting exactly where the file put it.
+        #expect(card.document.labels.value != nil)
     }
 
     @Test func everyIndexMdRoundTripsByteIdentically() throws {