refactor(session): fold the session family into packages/session/

git mv the 12 packages from session-persistence/, session-projection/,
session-title/, and telemetry/ into one session/ group per the
regrouping RFC; merge the four group READMEs into one bilingual
triplet; rewrite the group segment in tsconfig references (intra-group
references shorten to ../<pkg>), tsconfig.base.json paths/globs,
knip.json keys, vitest include, gate scripts, and authored doc/note
citations; regenerate module graph, doc graphs, catalogs, and the
lockfile importer keys. No npm names change.

Full unit suite: 8779 passed; the 18 reported failures reproduce as
env flakes (ambient-proxy IPv6 tunneling, watched-dir inotify
timeouts under parallel load) — each passes in isolation with
NO_PROXY set, matching their known pre-existing behavior on master.

packages/session/session-persistence/src/write-behind.ts

@@ -0,0 +1,159 @@
+/**
+ * Bounded per-session write batching for the shared persistence coordinator.
+ * @module @deepseek-ai/dsh-session-persistence/write-behind
+ */
+
+import type { SessionEvent } from '@deepseek-ai/dsh-session'
+
+/** Dependencies and scheduling policy for one live session's write controller. */
+export interface SessionWriteBehindOptions {
+  /** Maximum intentional batching wait after an idle queue receives work. */
+  readonly maxDelayMs: number
+  /** Persist one stable ordered prefix; resolves only after backend durability. */
+  readonly write: (events: readonly SessionEvent[]) => Promise<void>
+  /** Observe a detached background write failure without rejecting the producer. */
+  readonly reportBackgroundFailure: (error: unknown) => void
+}
+
+/**
+ * Owns one live session's pending events, fixed batching deadline, active write,
+ * failure retention, and explicit quiescence barrier.
+ */
+export class SessionWriteBehind {
+  private pending: SessionEvent[] = []
+  private timer: ReturnType<typeof setTimeout> | undefined
+  private active: Promise<void> | undefined
+  private barrier: Promise<void> | undefined
+  private deadlineExpired = false
+  private automaticPaused = false
+
+  /**
+   * @param options - fixed scheduling policy and durable batch sink.
+   */
+  constructor(private readonly options: SessionWriteBehindOptions) {}
+
+  /** Whether this controller owns queued events or an active durable write. */
+  get hasWork(): boolean {
+    return this.pending.length > 0 || this.active !== undefined
+  }
+
+  /**
+   * Copy one event into the persistence-owned queue and start a fixed deadline
+   * when the automatic path is idle.
+   * @param event - frozen live event to retain independently of its producer.
+   */
+  enqueue(event: SessionEvent): void {
+    const wasEmpty = this.pending.length === 0
+    this.pending.push(structuredClone(event))
+    if (this.barrier !== undefined) return
+    if (this.automaticPaused) {
+      this.automaticPaused = false
+      this.deadlineExpired = false
+      this.armTimer()
+    } else if (wasEmpty) {
+      this.armTimer()
+    }
+  }
+
+  /**
+   * Cancel the batching wait and durably drain through a quiescent point.
+   * Concurrent callers join the same barrier.
+   * @returns a promise that rejects if the barrier's durable retry fails.
+   */
+  flush(): Promise<void> {
+    if (this.barrier !== undefined) return this.barrier
+    this.cancelTimer()
+    this.deadlineExpired = false
+    this.automaticPaused = false
+    const barrier = Promise.withResolvers<void>()
+    this.barrier = barrier.promise
+    void this.drainBarrier(barrier.resolve, barrier.reject)
+    return barrier.promise
+  }
+
+  /** Cancel the current automatic deadline without draining retained work. */
+  cancelAutomaticWait(): void {
+    this.cancelTimer()
+    this.deadlineExpired = false
+  }
+
+  /** Start the one fixed window for the current pending prefix. */
+  private armTimer(): void {
+    this.timer = setTimeout(() => { this.onDeadline() }, this.options.maxDelayMs)
+  }
+
+  /** Cancel any pending automatic deadline. */
+  private cancelTimer(): void {
+    if (this.timer === undefined) return
+    clearTimeout(this.timer)
+    this.timer = undefined
+  }
+
+  /** Start a background write now, or remember that an active write used the budget. */
+  private onDeadline(): void {
+    this.timer = undefined
+    if (this.active !== undefined) {
+      this.deadlineExpired = true
+      return
+    }
+    this.startBackground()
+  }
+
+  /** Start one detached write whose failure is reported and retained. */
+  private startBackground(): void {
+    const active = this.startWrite(true)
+    void active.then(() => { this.continueAutomatic() }, () => {})
+  }
+
+  /** Continue immediately after an over-budget active write, otherwise keep its timer. */
+  private continueAutomatic(): void {
+    if (this.barrier !== undefined || this.pending.length === 0) return
+    if (this.deadlineExpired) {
+      this.deadlineExpired = false
+      this.startBackground()
+    }
+  }
+
+  /** Await overlapping work, drain to quiescence, and settle the shared barrier. */
+  private async drainBarrier(resolve: () => void, reject: (reason?: unknown) => void): Promise<void> {
+    try {
+      const overlapping = this.active
+      if (overlapping !== undefined) {
+        await Promise.allSettled([overlapping])
+        this.automaticPaused = false
+      }
+      while (this.pending.length > 0) await this.startWrite(false)
+    } catch (error: unknown) {
+      this.barrier = undefined
+      reject(error)
+      return
+    }
+    // Close admission to this barrier in the same job that observes the empty
+    // queue, before resolving callers. A later enqueue therefore starts its own
+    // automatic window instead of being stranded behind a settled barrier.
+    this.barrier = undefined
+    resolve()
+  }
+
+  /** Start one stable pending prefix, retaining it in order if durability fails. */
+  private startWrite(background: boolean): Promise<void> {
+    const batch = this.pending.splice(0)
+    this.cancelTimer()
+    this.deadlineExpired = false
+    const operation = Promise.resolve().then(() => this.options.write(batch))
+    const active = operation
+      .catch((error: unknown) => {
+        this.pending = batch.concat(this.pending)
+        this.cancelTimer()
+        this.deadlineExpired = false
+        this.automaticPaused = true
+        if (background) this.options.reportBackgroundFailure(error)
+        throw error
+      })
+      .finally(() => {
+        this.active = undefined
+      })
+    this.active = active
+    return active
+  }
+}