Comments, phase 3 — search, the thread find, announcements, and a11y

Board search reaches comment bodies through a search-owned transient
index: the first live-query keystroke sweeps comments/*/index.md
off-actor (.draft and comments/.trash excluded), keystrokes re-filter
in memory, the index discards on clear — the snapshot stays O(cards).
⌘F routes by focus: the comments pane gets an app-owned find bar
spanning the whole rendered thread (next/prev cross rows with
wraparound); body and composer keep NSTextFinder; Find Next/Previous
graduate from FutureCommands. Foreign comment changes speak
path-shaped beside the announcer's ladder ("New comment on 'X'",
plural folds), narrowed by EchoLedger receipts consumed through
CommentPath.classify — and that read fixed a latent footprint bug
where a comment receipt resolved against the card's attachment
listing, read .absent, and classified the user's own write as
foreign. The pane completes its a11y story: flattened comment
elements with Edit/Delete/Reveal custom actions (un-flattening
during inline edit), phrase-table vocabulary, labeled composer and
sort control, and an audit over the open pane on a comment-seeded
fixture (runnable only where automation permission exists).

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/UI/Card/CardWindowMetrics.swift

@@ -167,6 +167,15 @@ enum CardWindowMetrics {
         (lineHeight(bodyPointSize: bodyPointSize) * 5.5).rounded()
     }
 
+    /// The find bar's query field — **twelve characters**, the shortest measure that still shows a
+    /// two-word search whole. It is deliberately narrower than the board's search field (seventeen):
+    /// this bar shares a strip with a counter and three controls inside a fixed-width pane, and a
+    /// field sized to the query would push the Done button off the end of the narrowest window the
+    /// design allows.
+    static func findFieldWidth(bodyPointSize: CGFloat) -> CGFloat {
+        columnWidth(characters: 12, bodyPointSize: bodyPointSize)
+    }
+
     /// The gap between two comments in the thread — a full gutter, one step larger than the rhythm
     /// *inside* a comment (`sidebarRowSpacing`), so the eye groups an author line with its body
     /// rather than with its neighbour.