Give transient UI state an explicit home in the store

TransientBoardState, one per store, holds state by how a reload treats
it: item-referencing sets (selection, drag membership, pending cut)
share one shape and one constraint rule — members must exist in the
current universe — applied in two directions by one primitive, so the
search filter's hidden-cards rule and reload survival are one rule
expressed once; derived state is stored as its inputs only (the query,
never its result set); and the new-card placeholder is a lane-anchored
overlay with no UUID until commit, discarded when its lane vanishes or
tombstones, handed off when the created card's UUID appears. Trash
visibility rides along per-open, never persisted. The decision is
written back into DESIGN/02 § Changes from Kanban — the TBD is closed.

Full suite 352 tests in 64 suites green. Two findings filed.

Claude-Session: https://claude.ai/code/session_018BjQRYBR6jQja3jCRi5S3A

KanbanTests/BoardLoaderTests.swift

@@ -448,7 +448,7 @@ struct ItemIDValueSemanticsTests {
     }
 
     /// The consequence every `Set`/`Dictionary` keyed by `ItemID` inherits — selection
-    /// membership included (`BoardStore.Selection`).
+    /// membership included (`ItemReferenceSet`).
     @Test func aSetCollapsesTheTwoSpellingsToOneMember() {
         let set: Set<ItemID> = [ItemID(rawValue: Self.lower), ItemID(rawValue: Self.upper)]
         #expect(set.count == 1)