perf: reuse caller stat in fileBelongsToTeam, drop duplicate fs.stat

On the live resolution path collectRootJsonlSessionIds already stat()s each root
jsonl for its mtime-window filter, then fileBelongsToTeam stat()ed the very same
file again for its cache validation -- two fs.stat syscalls (plus two Stats
allocations) per file, every poll. fileBelongsToTeam now takes an optional
precomputed stat and the mtime-filter caller passes the stat it already has, so the
file is statted once. Measured 20 files -> 20 stat calls on the mtime path (was ~40).

Using a single stat snapshot is also slightly more consistent than two reads that
could straddle a concurrent write. The other call site (subagent scan) passes no
stat and is unchanged (fileBelongsToTeam stats it itself). Adds a regression test
that a caller-supplied stat is the one recorded in the affinity cache.

src/main/services/team/TeamTranscriptProjectResolver.ts

@@ -966,17 +966,19 @@ export class TeamTranscriptProjectResolver {
       while (nextIndex < rootJsonlEntries.length) {
         const entry = rootJsonlEntries[nextIndex++];
         const filePath = path.join(projectDir, entry.name);
+        let precomputedStat: { mtimeMs: number; size: number; isFile: () => boolean } | undefined;
         if (mtimeSinceMs != null) {
           try {
             const stat = await fs.stat(filePath);
             if (!stat.isFile() || stat.mtimeMs < mtimeSinceMs) {
               continue;
             }
+            precomputedStat = stat;
           } catch {
             continue;
           }
         }
-        if (!(await this.fileBelongsToTeam(filePath, teamName))) {
+        if (!(await this.fileBelongsToTeam(filePath, teamName, precomputedStat))) {
           continue;
         }
         discovered.add(entry.name.slice(0, -'.jsonl'.length));
@@ -1008,17 +1010,29 @@ export class TeamTranscriptProjectResolver {
     return this.collectRootJsonlSessionIds(rootJsonlEntries, projectDir, teamName, mtimeSinceMs);
   }
 
-  private async fileBelongsToTeam(filePath: string, teamName: string): Promise<boolean> {
+  private async fileBelongsToTeam(
+    filePath: string,
+    teamName: string,
+    precomputedStat?: { mtimeMs: number; size: number; isFile: () => boolean }
+  ): Promise<boolean> {
     const normalizedTeam = teamName.trim().toLowerCase();
     if (!normalizedTeam) {
       return false;
     }
 
+    // Reuse the caller's stat when it already statted this exact file (the mtime-window
+    // filter in collectRootJsonlSessionIds does). On the live resolution path this drops
+    // a second fs.stat of the same file per entry, every poll — and using a single stat
+    // snapshot is also more consistent than two reads that could straddle a write.
     let fileStat: { mtimeMs: number; size: number; isFile: () => boolean };
-    try {
-      fileStat = await fs.stat(filePath);
-    } catch {
-      return false;
+    if (precomputedStat) {
+      fileStat = precomputedStat;
+    } else {
+      try {
+        fileStat = await fs.stat(filePath);
+      } catch {
+        return false;
+      }
     }
 
     if (!fileStat.isFile()) {

test/main/services/team/TeamTranscriptProjectResolver.test.ts

@@ -637,7 +637,11 @@ describe('TeamTranscriptProjectResolver', () => {
     headWindowFull: boolean;
   };
   type ResolverProbe = {
-    fileBelongsToTeam: (filePath: string, teamName: string) => Promise<boolean>;
+    fileBelongsToTeam: (
+      filePath: string,
+      teamName: string,
+      precomputedStat?: { mtimeMs: number; size: number; isFile: () => boolean }
+    ) => Promise<boolean>;
     buildTeamAffinityFileCacheKey: (filePath: string, normalizedTeam: string) => string;
     teamAffinityFileCache: Map<string, AffinityCacheEntry>;
   };
@@ -716,4 +720,33 @@ describe('TeamTranscriptProjectResolver', () => {
     expect(second?.belongsToTeam).toBe(true);
     expect(second!.size).toBeGreaterThan(sizeAfterFirst); // re-scanned + re-cached
   });
+
+  // Regression: when the caller already statted the file (the mtime-window filter in
+  // collectRootJsonlSessionIds), fileBelongsToTeam must reuse that stat rather than
+  // issuing a second fs.stat of the same file. Proven without mocking fs: a precomputed
+  // stat with a deliberately distinct size/mtime must be the one recorded in the cache.
+  it('reuses a caller-supplied stat instead of re-statting the file', async () => {
+    await setupClaudeRoot();
+    const resolver = new TeamTranscriptProjectResolver() as unknown as ResolverProbe;
+    const team = 'absent-team';
+    const projectDir = path.join(tmpDir!, 'projects', encodePath('/repo/precomp'));
+    await fs.mkdir(projectDir, { recursive: true });
+    const jsonlPath = path.join(projectDir, 'f.jsonl');
+    await fs.writeFile(
+      jsonlPath,
+      `${Array.from({ length: 45 }, (_, i) =>
+        JSON.stringify({ type: 'user', message: { role: 'user', content: `x ${i}` } })
+      ).join('\n')}\n`,
+      'utf8'
+    );
+
+    // Distinct sentinel values the real file does not have.
+    const precomputedStat = { mtimeMs: 123_456, size: 999_999, isFile: () => true };
+    expect(await resolver.fileBelongsToTeam(jsonlPath, team, precomputedStat)).toBe(false);
+
+    const key = resolver.buildTeamAffinityFileCacheKey(jsonlPath, team);
+    const entry = resolver.teamAffinityFileCache.get(key);
+    expect(entry?.size).toBe(999_999); // cache recorded the precomputed stat -> no re-stat
+    expect(entry?.mtimeMs).toBe(123_456);
+  });
 });