Add support for Edge Side Includes (ESI)

Support [ESI (Edge Side Includes)](https://www.w3.org/TR/esi-lang/) in Harper's HTTP response layer. ESI enables HTML composition at the edge — sub-resources are fetched and injected inline when serving a response, allowing page fragments to be cached independently at different TTLs and assembled close to the data.

## Relevance to Harper

Harper's position as both a data layer and an HTTP server (with caching) makes it a natural host for ESI: fragments can be served directly from Harper data, composed server-side, and cached at the appropriate scope.

## Design considerations (Not Ready — needs scoping)

- Which ESI tags? At minimum `` and ``.
- Processing pipeline: pre-response transform pass over HTML bodies.
- Interaction with existing cache headers — fragment TTLs vs. page TTLs.
- Error handling: `/` support for resilient fallbacks.

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🤖 Filed by Claude on behalf of Kris.