elelem v0.2.0 — prompts become one immutable value; multimodal content Breaking: Message.Content is Content ([]Part), not string. Breaking: the system-message and history builders move off Request onto a new immutable Prompt, handed over in one WithPrompt call. Adds image, audio and document content parts, local refusal of content the model cannot read (ErrUnsupportedContent), the three new content capabilities, and WithCapabilityOverride for a driver aimed at a compatible gateway. Fixes cloneMessages not copying content, which with byte payloads left the engine transcript aliasing the caller's image buffer.