fix(tui): preserve high-water mark in sidebar, add copy_cost_from helper

Sidebar cost total now uses displayed_session_cost_for_currency so the
high-water mark protects against cost reversal during API reconciliation
(#244). The session+agents breakdown is only shown when it matches the
displayed total; otherwise a single total line is rendered.

Add SessionMetadata::copy_cost_from to eliminate the last remaining
manual four-field cost copy (fork-session path).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/tui/src/main.rs

@@ -2797,10 +2797,7 @@ fn fork_session(session_id: Option<String>, last: bool, workspace: &Path) -> Res
         saved.metadata.total_tokens,
         system_prompt.as_ref(),
     );
-    forked.metadata.session_cost_usd = saved.metadata.session_cost_usd;
-    forked.metadata.session_cost_cny = saved.metadata.session_cost_cny;
-    forked.metadata.subagent_cost_usd = saved.metadata.subagent_cost_usd;
-    forked.metadata.subagent_cost_cny = saved.metadata.subagent_cost_cny;
+    forked.metadata.copy_cost_from(&saved.metadata);
     manager.save_session(&forked)?;
 
     let source_title = saved.metadata.title.trim();

crates/tui/src/session_manager.rs

@@ -110,6 +110,16 @@ pub struct SessionMetadata {
     pub subagent_cost_cny: f64,
 }
 
+impl SessionMetadata {
+    /// Copy cost fields from another metadata (used when forking a session).
+    pub fn copy_cost_from(&mut self, other: &SessionMetadata) {
+        self.session_cost_usd = other.session_cost_usd;
+        self.session_cost_cny = other.session_cost_cny;
+        self.subagent_cost_usd = other.subagent_cost_usd;
+        self.subagent_cost_cny = other.subagent_cost_cny;
+    }
+}
+
 /// A saved session containing full conversation history
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct SavedSession {

crates/tui/src/tui/sidebar.rs

@@ -671,16 +671,25 @@ fn render_context_panel(f: &mut Frame, area: Rect, app: &App) {
     )));
 
     // ── Session cost ─────────────────────────────────────────────
+    let displayed_total = app.displayed_session_cost_for_currency(app.cost_currency);
     let session_cost = app.session_cost_for_currency(app.cost_currency);
     let agent_cost = app.subagent_cost_for_currency(app.cost_currency);
-    let total_cost = session_cost + agent_cost;
-    lines.push(Line::from(Span::styled(
+    let real_total = session_cost + agent_cost;
+    // Only show the additive breakdown when it matches the displayed
+    // total; when the high-water mark is in effect (post-reconciliation),
+    // the breakdown would not sum to the displayed value (#244).
+    let cost_line = if (displayed_total - real_total).abs() < 1e-9 {
         format!(
             "cost: {} (session {} + agents {})",
-            app.format_cost_amount(total_cost),
+            app.format_cost_amount(displayed_total),
             app.format_cost_amount(session_cost),
             app.format_cost_amount(agent_cost)
-        ),
+        )
+    } else {
+        format!("cost: {}", app.format_cost_amount(displayed_total))
+    };
+    lines.push(Line::from(Span::styled(
+        cost_line,
         Style::default().fg(palette::TEXT_MUTED),
     )));