[Epic 4] Parallel Tool Execution #359

Part of #355.

Gated behind PARALLEL_TOOL_EXECUTION experiment flag (Story 1.3). Requires Stories 1.2 and 4.1 first. Stories within this epic are strictly sequential.

Stories

• Story 4.1 — Per-call ToolExecutionContext
• Story 4.2a — DispatchState machine refactor
• Story 4.2b — Core parallel dispatch in presentAssistantMessage
• Story 4.2c — File-level write serialization and observability
• Story 4.3 — Serialize approval dialogs for parallel tools

Dependencies

Requires Stories 1.2 and 1.3 from Epic 1. Story 4.1 can start as soon as those land.