feat(wire,kernel,conversation-store): step grouping via stepId for batched tool calls

Expose a per-step grouping key so a client can render a model's batched/parallel
tool calls (those emitted in one step) as one unit, on both the live stream and
replayed history. Key = branded StepId, derived turnId#stepIndex (0-based).

- [email protected]: required stepId on Turn{Tool,ToolResult}Event; optional stepId on
  Tool{Call,Result}Chunk (generation provenance on the chunk, not the StoredChunk
  envelope — StoredChunk unchanged). [email protected] (re-export bump).
- kernel-runtime: mint stepId per step; stamp on tool chunks + tool events.
- conversation-store: chunk-carried stepId round-trips append/load/loadSince for
  free; reconcile copies it onto synthesized (interrupted) results.
- cli: stepId added to event test fixtures (renderer unchanged).

typecheck clean; 509 vitest + 89 bun; biome 0/0. FE courier reply + reference
snapshots regenerated in ../dispatch-web.

1 files changed, 14 insertions, 2 deletions

diff --git a/packages/kernel/src/runtime/events.ts b/packages/kernel/src/runtime/events.ts
index a209b00..deeb012 100644
--- a/packages/kernel/src/runtime/events.ts
+++ b/packages/kernel/src/runtime/events.ts
@@ -1,3 +1,4 @@
+import type { StepId } from "../contracts/conversation.js";
 import type { AgentEvent } from "../contracts/events.js";
 import type { Usage } from "../contracts/provider.js";
 
@@ -16,22 +17,33 @@ export function reasoningDeltaEvent(
 export function toolCallEvent(
 	conversationId: string,
 	turnId: string,
+	stepId: StepId,
 	toolCallId: string,
 	toolName: string,
 	input: unknown,
 ): AgentEvent {
-	return { type: "tool-call", conversationId, turnId, toolCallId, toolName, input };
+	return { type: "tool-call", conversationId, turnId, stepId, toolCallId, toolName, input };
 }
 
 export function toolResultEvent(
 	conversationId: string,
 	turnId: string,
+	stepId: StepId,
 	toolCallId: string,
 	toolName: string,
 	content: string,
 	isError: boolean,
 ): AgentEvent {
-	return { type: "tool-result", conversationId, turnId, toolCallId, toolName, content, isError };
+	return {
+		type: "tool-result",
+		conversationId,
+		turnId,
+		stepId,
+		toolCallId,
+		toolName,
+		content,
+		isError,
+	};
 }
 
 export function toolOutputEvent(