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/UI/Board/LaneView.swift

@@ -583,11 +583,17 @@ struct LaneView: View {
     /// layout that re-admitted them on every flip would flap the board under the cursor
     /// (DRAG-REORDER.md § Resting-layout zones). The copy's originals reappear when the write lands.
     ///
-    /// This is also the collection m5's search filter narrows, which is what keeps the count badge
-    /// honest for free — see `countBadge`.
+    /// **A card the live search filter hides renders nowhere either** (04-interactions.md § Search):
+    /// "cards whose title *and* body both miss the query animate out". This is the one collection
+    /// that narrowing, which is what makes the filter "the single source of truth for what's on the
+    /// board" true of this lane's every surface at once — the masonry, the count badge (see
+    /// `countBadge`), the drop zones' resting layout, the marquee registration and the Finder
+    /// file-drop targets all read this list or the registry it populates, so none of them needs a
+    /// rule of its own.
     private var renderedCards: [Card] {
         let hidden = drops.session.hiddenMembers(onBoardRooted: store.rootURL)
-        return lane.cards.filter { !$0.isDeleted && !hidden.contains($0.id) }
+        let filter = store.searchFilter
+        return lane.cards.filter { !$0.isDeleted && !hidden.contains($0.id) && filter.matches($0) }
     }
 
     // MARK: - Selection