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/SelectionClicks.swift

@@ -91,6 +91,19 @@ extension View {
     /// The frame is measured in `BoardView.stripSpace`, the one space every marquee coordinate lives
     /// in — the band's own points come from a drag gesture in the same space, so no conversion
     /// happens anywhere.
+    ///
+    /// **This is also how the search filter reaches the band and the arrows** (04-interactions.md
+    /// § Search, "marquee, … arrow nav … all read it"): a card the filter hides is never built, so
+    /// it registers nothing, and the two surfaces that navigate by drawn frames narrow with the
+    /// masonry rather than re-running the predicate.
+    ///
+    /// One bounded honesty about that: a card leaving under the filter's transition stays registered
+    /// until the transition ends (`onDisappear` fires when the view really goes, not when the query
+    /// changed), so for the length of one content-reflow spring a fading card is still sweepable and
+    /// still an arrow's neighbour. It is on screen for exactly that span, and it has already left the
+    /// selection (`TransientBoardState.constrainToSearch(in:)` runs at the keystroke), so the window
+    /// is visible rather than phantom — accepted rather than closed by teaching three input sites a
+    /// predicate the layout already applied.
     @MainActor
     func marqueeTarget(
         _ id: ItemID,