v0.4.0: differentiator — prose for TUI-side (on-daemon-msg, status bar, add-msg)

Prose added:
- gateway-tui-main.org: Contract item 2 updated — on-daemon-msg now
  extracts :gate-trace, :rule-count, :foveal-id from daemon response
- gateway-tui-view.org: Status Bar section — explains the three
  differentiator visualizations (rule counter, focus map, gate trace),
  noting they cost 0 LLM tokens and are unique to Passepartout's
  deterministic gate architecture
- gateway-tui-model.org: Contract item 2 updated — add-msg supports
  &key gate-trace for message-attached trace rendering

org/gateway-tui-main.org

@@ -17,7 +17,10 @@ Event handlers + daemon I/O + main loop.
    chat and history. Non-printable keys are ignored.
 2. (on-daemon-msg msg): processes inbound daemon messages. Routes
    text responses to chat display (:agent), handshake to system
-   messages, routes errors to log via ~log-message~.
+   messages, routes errors to log via ~log-message~. Extracts
+   ~:gate-trace~ (attached to message), ~:rule-count~, and
+   ~:foveal-id~ (v0.4.0 differentiator) from daemon response and
+   updates TUI state for status bar rendering.
 3. (send-daemon msg): serializes and sends a message to the daemon
    over the framed TCP protocol.
 4. (tui-main): the main loop — connects to daemon, initializes