Merge latest master into invariant service seam

docs/core-data-structures/session.md

@@ -430,7 +430,7 @@ declare class Session {
 - `context/message` → a user-role message carrying its `content` verbatim at its chronological position. Optional JSON `meta` remains in the event log and is never rendered.
 - `steering/message` → a user-role message carrying its content verbatim at its chronological position.
 
-Everything else (`turn/*`, `step/*`) is structural and does not project into a message. Token usage is observed on `assistant/message.usage` (the step that produced it); an operational error's step number is on `turn/end.reason` for `kind: 'error'`. Because this unreleased format intentionally has no compatibility promise, seed/load validation rejects request headers without provider+model and assistant messages without provider/model provenance instead of guessing a route for historical data.
+Everything else (`turn/*`, `step/*`, plugin-owned `llm/retry`) is structural and does not project into a message. Token accounting reads per-step `assistant/chunk { type: 'usage' }` records and treats `assistant/message.usage` as the committed-step fallback when no usage chunk exists; failed model-request attempts have no assistant message, so their usage chunk is the durable accounting record. An operational error's step number is on `turn/end.reason` for `kind: 'error'`, with normalized `LlmFailure` facts for a final model-request failure and message/code for other live errors. Because this unreleased format intentionally has no compatibility promise, seed/load validation rejects request headers without provider+model and assistant messages without provider/model provenance instead of guessing a route for historical data.
 
 ## Live-session fork API
 
@@ -474,9 +474,13 @@ interface TurnEndReasonMap {
    * The turn failed: a step threw or the model reported a failure. `step` is the
    * step number the failure occurred on (the operational error's location — the
    * single durable record of an in-turn failure; live diagnostics also fire via
-   * `agent/error`). `code` is the error's code when one was attached.
+   * `agent/error`). Final model-request failures retain their normalized facts
+   * as one `failure`; other turn failures retain their live Error message/code.
    */
-  error: { kind: 'error'; step: number; message: string; code?: string }
+  error: { kind: 'error'; step: number } & (
+    | { failure: LlmFailure; message?: never; code?: never }
+    | { message: string; code?: string; failure?: never }
+  )
   disposed: { kind: 'disposed' }
   /** At least one step reached its output-token ceiling, even if a plugin continued the turn. */
   'max-tokens': { kind: 'max-tokens' }