The LLM seam and its provider adapters. The interface package (`llm`) owns the abstract service, the content-block vocabulary, and the stream-chunk assembler; the adapters are concrete implementations that register on `ctx.llm`. All **product** packages.
Adapters register provider routes on the seam; retry and token measurement remain separate consumers. The child READMEs own routing, metadata, replay, and provider-wire details; the [LLM architecture decisions](../../.agents/notes/implemented/architecture/2026-06-13-twin-llm-adapters.md) own the rationale.