Break KVStorageComposite into focused classes

- KVReplicaSet: seed, apply, local scan, diff against the source, sync meta
- KVOutbox: persisted offline queue with in-order replay (load() resumes it)
- utils: KeyRange (also used for server authorization), KeyCounter,
  SerialQueue (also used by KVStorageHttp and the event-sourced store),
  RetryTimer (separate timers for sync and replay; previously one shared
  timer could cancel the other's retry)

The composite now only orchestrates. Behaviour unchanged; new unit tests for
the building blocks and the outbox.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Qxbkp4tA7G5SCWjzWuxmE

src/utils/key-range.ts

@@ -0,0 +1,36 @@
+import type { ScanOptions } from '../kv-storage/base-kv-storage.js'
+import { Keys } from './keys.js'
+
+/**
+ * A key and its whole subtree: `[prefix, prefix + '0')`. Escaping guarantees nothing else sorts
+ * in that range (not even lookalikes such as `prefix-x`). The empty prefix is every key.
+ */
+export class KeyRange {
+  /** Inclusive lower bound (`undefined` = unbounded). */
+  readonly gte: string | undefined
+  /** Exclusive upper bound (`undefined` = unbounded). */
+  readonly lt: string | undefined
+
+  constructor(readonly prefix: string) {
+    this.gte = prefix === '' ? undefined : prefix
+    this.lt = prefix === '' ? undefined : prefix + Keys.afterSeparator
+  }
+
+  contains(key: string): boolean {
+    return this.prefix === '' || key === this.prefix || key.startsWith(this.prefix + Keys.separator)
+  }
+
+  /** Whether a scan over `[gte, lt)` stays inside this range. */
+  covers(gte: string | undefined, lt: string | undefined): boolean {
+    if (this.prefix === '') return true
+    return gte !== undefined && lt !== undefined && gte >= this.gte! && lt <= this.lt!
+  }
+
+  /** Scan options narrowed to this range, or `undefined` if nothing is left. */
+  clamp(options: ScanOptions): ScanOptions | undefined {
+    const gte = this.gte !== undefined && (options.gte === undefined || options.gte < this.gte) ? this.gte : options.gte
+    const lt = this.lt !== undefined && (options.lt === undefined || options.lt > this.lt) ? this.lt : options.lt
+    if (gte !== undefined && lt !== undefined && gte >= lt) return undefined
+    return { ...options, gte, lt }
+  }
+}