fix(tui): move tolerance to module-level const, add total helpers

Promote COST_EQ_TOLERANCE from a function-local const to a module-level
constant in sidebar.rs.

Add SessionCostSnapshot::total_usd() and total_cny() helpers that
encapsulate session+subagent cost summation, used during session restore.

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

crates/tui/src/session_manager.rs

@@ -125,6 +125,18 @@ pub struct SessionCostSnapshot {
     pub displayed_cost_high_water_cny: f64,
 }
 
+impl SessionCostSnapshot {
+    /// Session + subagent cost in USD.
+    pub fn total_usd(&self) -> f64 {
+        self.session_cost_usd + self.subagent_cost_usd
+    }
+
+    /// Session + subagent cost in CNY.
+    pub fn total_cny(&self) -> f64 {
+        self.session_cost_cny + self.subagent_cost_cny
+    }
+}
+
 impl SessionMetadata {
     /// Copy cost fields from another metadata (used when forking a session).
     pub fn copy_cost_from(&mut self, other: &SessionMetadata) {

crates/tui/src/tui/sidebar.rs

@@ -26,6 +26,11 @@ use super::history::{HistoryCell, ToolCell, ToolStatus};
 use super::subagent_routing::active_fanout_counts;
 use super::ui::truncate_line_to_width;
 
+/// Tolerance for floating-point cost comparison in the sidebar breakdown.
+/// Must be large enough that accumulated f64 error across hundreds of turns
+/// does not prematurely hide the session+agents breakdown.
+const COST_EQ_TOLERANCE: f64 = 1e-6;
+
 pub fn render_sidebar(f: &mut Frame, area: Rect, app: &App) {
     if area.width < 24 || area.height < 8 {
         // Paint a styled block over the area so stale cells from a previous
@@ -678,9 +683,6 @@ fn render_context_panel(f: &mut Frame, area: Rect, app: &App) {
     // 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).
-    // Tolerance must be large enough that floating-point accumulation
-    // across many turns doesn't prematurely hide the breakdown.
-    const COST_EQ_TOLERANCE: f64 = 1e-6;
     let cost_line = if (displayed_total - real_total).abs() < COST_EQ_TOLERANCE {
         format!(
             "cost: {} (session {} + agents {})",

crates/tui/src/tui/ui.rs

@@ -6073,8 +6073,8 @@ fn apply_loaded_session(app: &mut App, session: &SavedSession) -> bool {
     // Take the max with the current totals — old sessions without
     // persisted high-water fields deserialise to 0.0 and fall back to
     // the restored total with no regression.
-    let total_restored_usd = app.session.session_cost + app.session.subagent_cost;
-    let total_restored_cny = app.session.session_cost_cny + app.session.subagent_cost_cny;
+    let total_restored_usd = session.metadata.cost.total_usd();
+    let total_restored_cny = session.metadata.cost.total_cny();
     app.session.displayed_cost_high_water =
         session.metadata.cost.displayed_cost_high_water_usd.max(total_restored_usd);
     app.session.displayed_cost_high_water_cny =