feat(web): turn speed metrics and composer context meter

Assistant footers and the stats line gain TTFT/tok-per-second readings
folded from step timings; context occupancy moves off the stats line onto
a composer ring whose panel shows a heuristic system/tools/messages
breakdown from the new token-meter contextBreakdown session projection.

docs/cordis-catalog/services.md

@@ -1681,7 +1681,7 @@ fork(source: SessionForkSource, boundary?: number, childSessionId?: SessionId):
 
 Types: [CreateSessionOptions](../core-data-structures/persistence.md) · [Session](../core-data-structures/session.md) · [SessionId](../core-data-structures/core.md)
 
-Source: [`packages/core/session/src/index.ts:767`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:726`](../../packages/core/session/src/index.ts)
 
 ## `ctx.sessionTitle` — `SessionTitleService`
 
@@ -2307,7 +2307,8 @@ Replay owner for one service-wide estimator and isolated per-session folds.
 measure(session: Session, requestHeader?: EpochHeader): TokenMeasurement
 
 /**
- * Heuristically price one model-visible message.
+ * Heuristically price one model-visible message (instance face of the pure
+ * {@link estimateMessage}).
  * @param message - message to price without mutation.
  * @returns content and role-framing tokens under the fixed service heuristic.
  */
@@ -2316,7 +2317,7 @@ estimateMessage(message: Message): number
 
 Types: [EpochHeader](../core-data-structures/session.md) · [Message](../core-data-structures/core.md) · [Session](../core-data-structures/session.md) · [TokenMeasurement](../core-data-structures/token-meter.md)
 
-Source: [`packages/llm/token-meter/src/index.ts:85`](../../packages/llm/token-meter/src/index.ts)
+Source: [`packages/llm/token-meter/src/index.ts:78`](../../packages/llm/token-meter/src/index.ts)
 
 ## `ctx.toolResultPrune` — `ToolResultPruneService`
 

docs/core-data-structures/session.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write docs/core-data-structures/session.md
-session.md: 6369c956df03c0d786db696c208b000300d5bfbc
-session.zh.md: c39382b7e6c9b14f91c311cc80526a6fd8898e4c
+session.md: 0d32b47585e01dd992167b6580245c5444f3891b
+session.zh.md: ad3fb474c4f9c68ef33bf9f1ddea0a1a64b31de6

docs/core-data-structures/session.md

@@ -487,15 +487,8 @@ declare class Session {
    */
   deriveMessages(): Message[];
   /**
-   * Project a single event into the LLM message it derives to, or null when
-   * it produces none — a non-surface event (chunk, boundary, log-only record)
-   * or an empty-content assistant/message (which exists only to host usage).
-   * The per-node pure function {@link deriveMessages} folds over the surface;
-   * an external reconstructor (or the dev invariant) folds the same function
-   * over a log prefix's surface to rebuild the exact messages any request was
-   * built from (the reconstructability Agent Note). The returned message is
-   * the already frozen message nested in the event wrapper and shared by
-   * delivery, durable history, and model requests.
+   * Instance face of the pure per-node projection rule
+   * {@link deriveEventMessage} (see its contract in `surface.ts`).
    * @param event - the event to project.
    * @returns the derived message, or null when the event produces none.
    */

docs/core-data-structures/session.zh.md

@@ -489,15 +489,8 @@ declare class Session {
    */
   deriveMessages(): Message[];
   /**
-   * Project a single event into the LLM message it derives to, or null when
-   * it produces none — a non-surface event (chunk, boundary, log-only record)
-   * or an empty-content assistant/message (which exists only to host usage).
-   * The per-node pure function {@link deriveMessages} folds over the surface;
-   * an external reconstructor (or the dev invariant) folds the same function
-   * over a log prefix's surface to rebuild the exact messages any request was
-   * built from (the reconstructability Agent Note). The returned message is
-   * the already frozen message nested in the event wrapper and shared by
-   * delivery, durable history, and model requests.
+   * Instance face of the pure per-node projection rule
+   * {@link deriveEventMessage} (see its contract in `surface.ts`).
    * @param event - the event to project.
    * @returns the derived message, or null when the event produces none.
    */