Migration: harn-hostlib host contracts

harn-hostlib began as a migration path for code-intelligence and tool surfaces that had lived in an external IDE host's Swift code-intelligence modules. That history explains the early parity tests and the initial schema names, but it is no longer the ownership model.

The current contract is Harn-owned:

• JSON schemas under crates/harn-hostlib/schemas/ define request and response compatibility for every hostlib method.
• HostlibRegistry is the authoritative runtime catalog of registered modules and methods.
• Consumer repositories should treat their bridge tests as compatibility checks against Harn's published contract, not as the source of truth for hostlib behavior.

During migration, keep any consumer-specific bridge notes in this page or in the consumer repository. Public hostlib docs, schema descriptions, and module comments should describe the neutral Harn contract first.