fix(storage): counts persistence is single-flight, coalesced, and never races its own temp file

persistCounts() was write-through on every count change with no
serialization, and the atomic writer named its temp file with millisecond
granularity. Two persists inside one millisecond shared the temp path: both
wrote it, the first rename consumed it, the second rename found nothing —
ENOENT, roughly 1,500 times a day on a busy production brain, with a full
ledger write per change behind it. No data was lost (the surviving rename
carried a complete ledger and the next change re-persisted), but the race
was real and the write rate absurd.

flushCounts() now runs exactly one persist at a time; requests arriving
during it collapse into one trailing pass that carries the burst's final
state — N changes cost at most two writes. writeFileAtomic() adds a
per-process sequence to the temp name so no two writes can share a path.
Pinned: a 25-change burst → ≤2 ledger writes, zero errors, ledger equal to
memory; parallel real writes land complete; three same-instant atomic
writes own three distinct temp paths.

src/storage/adapters/fileSystemStorage.ts

@@ -2400,8 +2400,15 @@ export class FileSystemStorage extends BaseStorage {
    * Atomic write via temp-file-then-rename so concurrent readers never see a
    * half-written lock JSON. Reused by writer-lock writes + heartbeat.
    */
+  /** Monotonic per-process sequence so two atomic writes never share a temp path. */
+  private static atomicWriteSeq = 0
+
   private async writeFileAtomic(filePath: string, contents: string): Promise<void> {
-    const tmp = `${filePath}.tmp-${process.pid}-${Date.now()}`
+    // pid + timestamp alone collided: two writers of the same target inside
+    // one millisecond shared this path, and the loser's rename found the
+    // winner had already moved it (ENOENT). The sequence makes every call's
+    // temp path its own.
+    const tmp = `${filePath}.tmp-${process.pid}-${Date.now()}-${++FileSystemStorage.atomicWriteSeq}`
     await fs.promises.writeFile(tmp, contents)
     await fs.promises.rename(tmp, filePath)
   }