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/BoardCommands.swift

@@ -79,6 +79,34 @@ extension BoardStore {
     }
 }
 
+/// **The caret-chords rule, as one expression** (04-interactions.md ▸ Grammar, settled):
+///
+/// > Board ▸ Move Left/Move Right ⌘←/⌘→ and the width pair ⌥⌘←/⌥⌘→ disable via menu validation
+/// > whenever *any* text control has keyboard focus — inline title editors, the board search field,
+/// > board-popover fields (rename, git identity, remote), and card-window fields — because an
+/// > enabled menu key equivalent fires before the field ever sees the key, and ⌘←/⌘→ are the
+/// > standard line-start/end caret chords.
+///
+/// Four text surfaces, answered four ways, and only two of them are here:
+///
+/// - **Inline title editors** are `acceptsBoardMutations`', through the focused-editor rule — a
+///   broader lockdown that already covers these two items.
+/// - **The board popover's fields** are covered by disabling while the popover is open at all —
+///   coarser than per-field focus, but it is a configuration surface (04's carve-out) and no lane
+///   move belongs under it.
+/// - **The search field** is per-focus and exact (`BoardSearchPresentation.isFocused`), which it has
+///   to be: the field's own rule is that board commands *stay enabled* while it holds the keyboard
+///   (04 § Search), so these two are the narrow exception to it and nothing coarser would do.
+/// - **Card-window fields** need nothing: those windows never publish a `boardStore`, so both items
+///   are already scopeless there.
+///
+/// Stated once because the two command groups must not drift: a rule with two implementations is a
+/// rule with two chances to forget a surface.
+@MainActor
+func caretChordsYield(boardInfo: BoardInfoPresentation?, search: BoardSearchPresentation?) -> Bool {
+    boardInfo?.isPresented == true || search?.isFocused == true
+}
+
 // MARK: - Open Card
 
 /// Board ▸ Open Card (⌘↩) — 11-command-nexus.md's first Board row, and **the one board command
@@ -205,32 +233,30 @@ struct MoveCardCommands: View {
 ///
 /// **Caret chords yield to any focused text control** (04-interactions.md ▸ Grammar, settled):
 /// ⌘←/⌘→ are the standard line-start/end chords, and an enabled key equivalent fires before a
-/// field ever sees the key. The inline title editors are covered by `acceptsBoardMutations`; the
-/// board popover's fields are covered by disabling while the popover is open at all — coarser than
-/// per-field focus, but the popover is a configuration surface (04's carve-out) and no lane move
-/// belongs under it. The search field (m5-search) and the card window's fields (whose windows never
-/// publish a `boardStore` in the first place) extend the same rule with their own cards.
+/// field ever sees the key. Which surfaces that covers, and how each is answered, is
+/// `caretChordsYield(boardInfo:search:)`'s doc comment — shared verbatim with the width pair below.
 struct MoveLaneCommands: View {
 
     @FocusedValue(\.boardStore) private var store
     @FocusedValue(\.boardInfo) private var boardInfo
+    @FocusedValue(\.boardSearch) private var search
 
     var body: some View {
         Button("Move Left") {
             move(by: -1)
         }
         .keyboardShortcut(.leftArrow, modifiers: .command)
-        .disabled(caretChordsYield || destination(-1) == nil)
+        .disabled(yieldsCaretChords || destination(-1) == nil)
 
         Button("Move Right") {
             move(by: 1)
         }
         .keyboardShortcut(.rightArrow, modifiers: .command)
-        .disabled(caretChordsYield || destination(1) == nil)
+        .disabled(yieldsCaretChords || destination(1) == nil)
     }
 
-    private var caretChordsYield: Bool {
-        boardInfo?.isPresented == true
+    private var yieldsCaretChords: Bool {
+        caretChordsYield(boardInfo: boardInfo, search: search)
     }
 
     /// The sole selected live lane and the display slot one step would put it in — `nil` when there
@@ -431,23 +457,24 @@ struct LaneWidthCommands: View {
 
     @FocusedValue(\.boardStore) private var store
     @FocusedValue(\.boardInfo) private var boardInfo
+    @FocusedValue(\.boardSearch) private var search
 
     var body: some View {
         Button("Increase Lane Width") {
             step(by: 1)
         }
         .keyboardShortcut(.rightArrow, modifiers: [.option, .command])
-        .disabled(caretChordsYield || selectedLanes.isEmpty)
+        .disabled(yieldsCaretChords || selectedLanes.isEmpty)
 
         Button("Decrease Lane Width") {
             step(by: -1)
         }
         .keyboardShortcut(.leftArrow, modifiers: [.option, .command])
-        .disabled(caretChordsYield || !canDecrease)
+        .disabled(yieldsCaretChords || !canDecrease)
     }
 
-    private var caretChordsYield: Bool {
-        boardInfo?.isPresented == true
+    private var yieldsCaretChords: Bool {
+        caretChordsYield(boardInfo: boardInfo, search: search)
     }
 
     /// The selected live lanes, in snapshot order — the batch, and the items' validation.