Implement live search filtering

The board's live title+body filter per 04-interactions.md § Search:

- SearchFilter — a pure value folding the query once (case- and
  diacritic-insensitive substring, locale-stable); title OR body matches,
  attachment filenames never searched; only the literal empty string is
  inactive.
- One universe: the filter threads through SelectionGrammar's order lists
  as a defaulted parameter, so ranges, Select All, arrow navigation, the
  marquee, drop zones, count badges, and the shown trash all read the same
  filtered set by construction; lanes are deliberately never filtered out
  (an emptied lane keeps its slot with a 0 badge). Hidden cards leave the
  selection through the existing constrain primitive, run on every query
  change and as the last line of the reload resolve; the delete successor
  is filtered so ⌫ never selects a hidden neighbour.
- The field: an NSSearchField-backed toolbar item (the toolbar's sole
  default item); Edit ▸ Find ⌘F focuses it through a focused-value
  presentation; stock field-editor dispatch — Return swallowed, Tab is the
  keep-filter path to the board, board commands stay enabled except the
  caret-chord pair, now one shared caretChordsYield expression.
- Escape is staged: clear the non-empty query (focus stays), hand an empty
  field back to the board, clear an active search from board focus —
  before Escape's clear-selection meaning.
- Creating a card clears the search (the placeholder funnel); a rename
  deliberately gets no carve-out; filter reflow rides the content spring
  keyed narrowly on the query.

903 unit tests (24 new).

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/App/BoardWindowHost.swift

@@ -50,6 +50,12 @@ struct BoardWindowHost: View {
     /// the board half of a card window's `(board, card)` identity — see `CardOpener`.
     @State private var cardOpener = CardOpener()
 
+    /// This window's toolbar search field, as a handle (`BoardSearchPresentation`). `@State` for
+    /// `boardInfo`'s reason — one per window — and published the same way, because Edit ▸ Find ⌘F
+    /// and the caret-chord commands are menu-bar items that have to reach the frontmost board
+    /// window's field.
+    @State private var boardSearch = BoardSearchPresentation()
+
     @State private var phase: Phase = .opening
 
     private enum Phase {
@@ -89,13 +95,32 @@ struct BoardWindowHost: View {
                     store: store,
                     window: { windowController.window },
                     confirmations: trashConfirmations,
-                    openCard: openCard
+                    openCard: openCard,
+                    search: boardSearch
                 )
             }
+            // **The board window's toolbar: the search field, nothing else** (03-board-ui.md ▸
+            // Toolbar, "trailing, the one default item; the titlebar stays clean"). It is a toolbar
+            // rather than a strip inside the content because that is where 03 puts it, and it hosts
+            // an `NSSearchField` rather than `.searchable` for the reasons `BoardSearchField`
+            // records — explicit first-responder control, and stock key behaviour.
+            //
+            // m6-toolbar: the rest of 03's toolbar story is the customization card's — the
+            // Customize palette, the New Card / New Lane / Undo / Redo / Show Trash catalog, and
+            // with it ⌘F's transient surfacing of a *removed* field. That work replaces this
+            // declaration with an identified, customizable toolbar; the item itself does not move.
+            .toolbar {
+                ToolbarItem(placement: .primaryAction) {
+                    BoardSearchField(store: store, presentation: boardSearch)
+                        .frame(width: 220)
+                }
+            }
             // "The board in front", for the menu items that act on it (`LaneWidthCommands`), and
-            // beside it the window's own popover flag, which is what File ▸ Board Info toggles, and
-            // its purge-alert host, which the trash's two confirmed commands raise.
+            // beside it the window's own popover flag, which is what File ▸ Board Info toggles, its
+            // purge-alert host, which the trash's two confirmed commands raise, and its search
+            // field, which Edit ▸ Find focuses and the caret-chord commands yield to.
             .focusedSceneValue(\.boardStore, store)
+            .focusedSceneValue(\.boardSearch, boardSearch)
             // The window's identity beside its store — File ▸ Duplicate flushes a *session*, which
             // is keyed on the window rather than on the board it is showing.
             .focusedSceneValue(\.boardWindowRef, ref)