Add server-side progress grouper and fix session cache race condition

New progress-grouper service partitions ParsedMessage arrays into two
outputs: a filtered messages list (orphaned progress stays inline) and
a toolProgress map keyed by parentToolUseId. Only hook_progress events
whose parentToolUseId matches an existing tool_call are extracted; all
others remain in the main message stream. Each group is sorted by
rawIndex for chronological display.

Session route integration:
- Pipe parseSession output through groupProgress before responding
- Return toolProgress map alongside messages in session detail endpoint

Cache improvements:
- Deduplicate concurrent getCachedSessions() calls with a shared
  in-flight promise (cachePromise) to prevent thundering herd on
  multiple simultaneous requests
- Track cache generation to avoid stale writes when a newer discovery
  supersedes an in-flight one
- Clear cachePromise on refresh=1 to force a fresh discovery cycle

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/server/routes/sessions.ts

@@ -1,6 +1,7 @@
 import { Router } from "express";
 import { discoverSessions } from "../services/session-discovery.js";
 import { parseSession } from "../services/session-parser.js";
+import { groupProgress } from "../services/progress-grouper.js";
 import type { SessionEntry } from "../../shared/types.js";
 
 export const sessionsRouter = Router();
@@ -8,13 +9,30 @@ export const sessionsRouter = Router();
 // Simple cache to avoid re-discovering sessions on every detail request
 let cachedSessions: SessionEntry[] = [];
 let cacheTimestamp = 0;
+let cachePromise: Promise<SessionEntry[]> | null = null;
+let cacheGeneration = 0;
 const CACHE_TTL_MS = 30_000;
 
 async function getCachedSessions(): Promise<SessionEntry[]> {
   const now = Date.now();
   if (now - cacheTimestamp > CACHE_TTL_MS) {
-    cachedSessions = await discoverSessions();
-    cacheTimestamp = now;
+    // Deduplicate concurrent calls: reuse in-flight promise
+    if (!cachePromise) {
+      const gen = ++cacheGeneration;
+      cachePromise = discoverSessions().then((sessions) => {
+        // Only write cache if no newer generation has started
+        if (gen === cacheGeneration) {
+          cachedSessions = sessions;
+          cacheTimestamp = Date.now();
+        }
+        cachePromise = null;
+        return sessions;
+      }).catch((err) => {
+        cachePromise = null;
+        throw err;
+      });
+    }
+    return cachePromise;
   }
   return cachedSessions;
 }
@@ -23,6 +41,7 @@ sessionsRouter.get("/", async (req, res) => {
   try {
     if (req.query.refresh === "1") {
       cacheTimestamp = 0;
+      cachePromise = null; // Discard any in-flight request so we force a fresh discovery
     }
     const sessions = await getCachedSessions();
     res.json({ sessions });
@@ -40,11 +59,13 @@ sessionsRouter.get("/:id", async (req, res) => {
       res.status(404).json({ error: "Session not found" });
       return;
     }
-    const messages = await parseSession(entry.path);
+    const allMessages = await parseSession(entry.path);
+    const { messages, toolProgress } = groupProgress(allMessages);
     res.json({
       id: entry.id,
       project: entry.project,
       messages,
+      toolProgress,
     });
   } catch (err) {
     console.error("Failed to load session:", err);

src/server/services/progress-grouper.ts

@@ -0,0 +1,42 @@
+import type { ParsedMessage } from "../../shared/types.js";
+
+export interface GroupedProgress {
+  messages: ParsedMessage[];
+  toolProgress: Record<string, ParsedMessage[]>;
+}
+
+export function groupProgress(messages: ParsedMessage[]): GroupedProgress {
+  // Build set of all toolUseId values from tool_call messages
+  const toolUseIds = new Set<string>();
+  for (const msg of messages) {
+    if (msg.category === "tool_call" && msg.toolUseId) {
+      toolUseIds.add(msg.toolUseId);
+    }
+  }
+
+  const filtered: ParsedMessage[] = [];
+  const toolProgress: Record<string, ParsedMessage[]> = {};
+
+  for (const msg of messages) {
+    // Parented progress: hook_progress with a parentToolUseId matching a known tool_call
+    if (
+      msg.category === "hook_progress" &&
+      msg.parentToolUseId &&
+      toolUseIds.has(msg.parentToolUseId)
+    ) {
+      if (!toolProgress[msg.parentToolUseId]) {
+        toolProgress[msg.parentToolUseId] = [];
+      }
+      toolProgress[msg.parentToolUseId].push(msg);
+    } else {
+      filtered.push(msg);
+    }
+  }
+
+  // Sort each group by rawIndex
+  for (const key of Object.keys(toolProgress)) {
+    toolProgress[key].sort((a, b) => a.rawIndex - b.rawIndex);
+  }
+
+  return { messages: filtered, toolProgress };
+}