docs: rename core-data-structures/ to subsystems/

The folder is becoming the home of one-doc-per-subsystem pages (intro +
data structures + cordis services/events), so the name must describe the
whole contract, not just the type-vocabulary third of it. Mechanical
rename rebuilt on current master: every inbound Markdown link, generator
constant, website route, type-equiv manifest path, and spec expectation
moves together; the zh sides of the notes whose prose names the folder
are aligned (子系统) in the same change; touched bilingual pairs
re-recorded; translation-prompt snapshot re-recorded (its example embeds
development.md). Historical Agent Note slugs keep their dated
filenames.

docs/subsystems/session-title.md

@@ -0,0 +1,146 @@
+# Session Titles
+
+English | [中文](session-title.zh.md)
+
+Durable latest-wins title state and the optional asynchronous provider vocabulary owned by [`@deepseek-ai/dsh-session-title`](../../packages/session/session-title). The shared LLM helper owns the exact auxiliary request record. Package READMEs own timing, fallback, failure, and fork behavior; the generated [persistence catalog](../persistence-catalog.md) owns the complete event declarations.
+
+Sources: [`packages/session/session-title/src/index.ts`](../../packages/session/session-title/src/index.ts), [`packages/session/session-title-llm/src/index.ts`](../../packages/session/session-title-llm/src/index.ts)
+
+## Durable title state
+
+`SessionTitleProviderId` is recorded for provider-produced revisions. `SessionTitleEventData` carries exact human-message provenance, while `SessionTitleSnapshot` adds the durable event envelope facts selected by `foldSessionTitle()`.
+
+```ts type-equiv
+/** Identifies one session-title provider registration. */
+type SessionTitleProviderId = Branded<'SessionTitleProviderId'>
+```
+
+```ts type-equiv
+/** Exact auxiliary model route that produced a title. */
+interface SessionTitleModelProvenance {
+  /** Registered LLM provider route. */
+  readonly provider: string
+  /** Provider model id. */
+  readonly model: string
+}
+```
+
+```ts type-equiv
+/** Durable ownership record for an accepted session title. */
+type SessionTitleSource =
+  | { readonly kind: 'fallback' }
+  | {
+    readonly kind: 'provider'
+    readonly provider: SessionTitleProviderId
+    readonly model?: SessionTitleModelProvenance
+  }
+  | {
+    /** Explicit user rename: pins the title — automatic generation stops scheduling. */
+    readonly kind: 'user'
+  }
+```
+
+```ts type-equiv
+/** Payload of the log-only `session/title` event. */
+interface SessionTitleEventData {
+  /** Normalized non-empty title text. */
+  readonly title: string
+  /** Exact human `user/message` seqs used to derive this title; empty for an explicit user rename. */
+  readonly messageSeqs: number[]
+  /** Built-in fallback, registered-provider, or explicit-user provenance. */
+  readonly source: SessionTitleSource
+}
+```
+
+```ts type-equiv
+/** Latest folded title plus the title event's durable envelope facts. */
+interface SessionTitleSnapshot extends SessionTitleEventData {
+  /** Seq of the latest `session/title` event. */
+  readonly eventSeq: number
+  /** Timestamp of the latest `session/title` event. */
+  readonly updatedAt: number
+}
+```
+
+## Auxiliary request record
+
+The shared LLM helper records each validated, dispatchable title request before calling the model. The payload reproduces the model-visible system and message input, routing, output limit, provider ownership, and source-message attribution even when generation later fails.
+
+```ts type-equiv
+/** Exact model-visible request recorded before one auxiliary title dispatch. */
+interface SessionTitleLlmRequestEventData {
+  /** Registered title-provider identity responsible for the request. */
+  readonly titleProvider: SessionTitleProviderId
+  /** Exact human `user/message` seqs represented in `messages`. */
+  readonly messageSeqs: number[]
+  /** Exact auxiliary LLM route. */
+  readonly route: SessionTitleModelProvenance
+  /** Exact auxiliary system prompt. */
+  readonly system: string
+  /** Exact auxiliary message list. */
+  readonly messages: Message[]
+  /** Exact auxiliary output-token cap. */
+  readonly maxTokens: number
+}
+```
+
+## Provider input and output
+
+The service snapshots eligible messages through one revision. A provider returns only seqs from that request; service-owned acceptance verifies ordering, normalizes the title, enforces the byte limit, and appends provenance.
+
+```ts type-equiv
+/** One eligible human text message exposed to title providers. */
+interface SessionTitleUserMessage {
+  /** Source `user/message` event seq. */
+  readonly seq: number
+  /** Exact concatenated text-block content. */
+  readonly text: string
+}
+```
+
+```ts type-equiv
+/** Automatic generation cadence owned by a registered provider. */
+type SessionTitleAutomaticMode = 'first-message' | 'all-user-messages'
+```
+
+```ts type-equiv
+/** Immutable input supplied to one title-provider call. */
+interface SessionTitleProviderRequest {
+  /** Live session being titled. */
+  readonly session: Session
+  /** All eligible human messages through this generation revision. */
+  readonly messages: readonly SessionTitleUserMessage[]
+  /** Exact current logged main-request route, when one has been recorded. */
+  readonly route?: SessionTitleModelProvenance
+  /** Cancellation for supersession, disposal, timeout composition, or the explicit caller. */
+  readonly signal: AbortSignal
+}
+```
+
+```ts type-equiv
+/** Provider output before service-owned normalization and log acceptance. */
+interface SessionTitleProviderResult {
+  /** Proposed title text. */
+  readonly title: string
+  /** Exact seqs from `request.messages` used by this result. */
+  readonly messageSeqs: readonly number[]
+  /** Auxiliary LLM route, when generation used a model. */
+  readonly model?: SessionTitleModelProvenance
+}
+```
+
+```ts type-equiv
+/** One optional asynchronous title implementation registered with the service. */
+interface SessionTitleProvider {
+  /** Stable provider identity recorded in title provenance. */
+  readonly id: SessionTitleProviderId
+  /** When new human prompts start automatic generation. */
+  readonly automatic: SessionTitleAutomaticMode
+  /**
+   * Produce one title revision.
+   * @param request - message snapshot, current route, session, and cancellation.
+   * @returns proposed title plus exact input seqs and optional model provenance.
+   */
+  generate(request: SessionTitleProviderRequest): Promise<SessionTitleProviderResult>
+}
+```