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/Motion.swift

@@ -109,8 +109,14 @@ enum Motion {
     /// restore reads like the search filter — leavers and arrivers run their transition, survivors
     /// reflow under one gentle spring".
     ///
-    /// Named ahead of both its call sites (m5's search, m7's undo) for the same reason `delete` is:
-    /// the figure is settled, and a duration that has nowhere to live gets spelled at a call site.
+    /// **The search filter is its call site** (`BoardView.laneStrip`), where it wraps a transaction
+    /// keyed on the query and nothing else — 03's own narrow key for this reflow. The leavers and
+    /// arrivers it talks about are `cardTransition`, already attached to every card slot and trash
+    /// row, so the two halves of the sentence are two modifiers rather than one bespoke animation.
+    ///
+    // m7-undo: the restore is the other half of the pair. It arrives as a bracketed wholesale
+    // reload, so it reaches this voice through `reloadAnimation` rather than through a transaction
+    // of its own — see `reloadAnimates(origin:endsBracketedOperation:)`.
     static func contentReflow(reduced: Bool) -> Animation? {
         reduced ? nil : .smooth(duration: Duration.contentReflow)
     }
@@ -145,8 +151,9 @@ enum Motion {
         reduced ? .crossfade : .scaleAndFade(from: AppearScale.lane)
     }
 
-    /// A card arriving or leaving — a create, a delete, a Put Back, and (m5) a search filter's
-    /// leavers and arrivers. The trash's rows wear it too: they are cards, and 10 requires the trash
+    /// A card arriving or leaving — a create, a delete, a Put Back, and the search filter's leavers
+    /// and arrivers ("Search-hiding rides the same structural transition — hiding is removal, not a
+    /// special fade"). The trash's rows wear it too: they are cards, and 10 requires the trash
     /// animations to have a reduced variant like everything else.
     static func cardTransition(reduced: Bool) -> AnyTransition {
         cardAppearance(reduced: reduced).transition
@@ -243,12 +250,15 @@ enum Motion {
     /// The thin wrapper `BoardStore.land` hands to `withAnimation`: the voice a landing snapshot is
     /// applied in, or `nil` for the reloads that snap.
     ///
-    // m5-drag, m5-search: the voice is the *general* structural spring for every app-mediated
-    // reload, because the reload seam knows an operation echoed but not which one — 03 gives delete
-    // 0.25 s and a drop commit its own dialect, and neither is reachable from an origin tag. The
-    // per-operation figures (`delete`, `dragReflow`) become reachable when the operations that own
-    // them run their own transactions around the gesture, which is m5's card; this seam stays the
-    // floor under them.
+    // m5-drag: the voice is the *general* structural spring for every app-mediated reload, because
+    // the reload seam knows an operation echoed but not which one — 03 gives delete 0.25 s and a
+    // drop commit its own dialect, and neither is reachable from an origin tag. The per-operation
+    // figures (`delete`, `dragReflow`) become reachable when the operations that own them run their
+    // own transactions around the gesture; this seam stays the floor under them.
+    //
+    // The search filter needed none of that and never reaches here: a query change is not a reload
+    // at all — it is transient state, so its transaction is wrapped where it happens
+    // (`BoardView.laneStrip`, `contentReflow`).
     static func reloadAnimation(origin: WatchOrigin, endsBracketedOperation: Bool, reduced: Bool) -> Animation? {
         guard reloadAnimates(origin: origin, endsBracketedOperation: endsBracketedOperation) else { return nil }
         return structural(reduced: reduced)