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/KanbanApp.swift

@@ -159,6 +159,19 @@ struct KanbanApp: App {
             TrashCommands()
         }
 
+        // The Edit menu's one row of ours: Find (⌘F), placed after the standard Cut/Copy/Paste/
+        // Select All group, which is where macOS puts Find. Undo/Redo and the clipboard items are
+        // the system's and the board answers them as a responder (`ClipboardCommands.swift`) — a
+        // second item sharing one of those titles is what titles-are-API forbids.
+        //
+        // m6-card-window: Find Next / Find Previous (⌘G/⇧⌘G) join here, scoped to the card window's
+        // find bar and "disabled in the board window — board search is a live filter, not a cursor"
+        // (11-command-nexus.md). They wait for the window that owns them rather than shipping as two
+        // permanently disabled rows.
+        CommandGroup(after: .pasteboard) {
+            FindCommand()
+        }
+
         // The View menu. `CommandGroupPlacement.toolbar` *is* View — the menu the toolbar's own
         // items live in — which is where 11-command-nexus.md files Show Trash, alongside the card
         // window's Edit Body / Raw Source / History still to come.