v0.4.0: Add thin root facade to preserve import compatibility

[tazarov]

Goal

Keep the root module import path stable by introducing a minimal root facade package that re-exports the moved implementation.

Scope

• Root package exposes the current public API surface only.
• Root delegates to relocated implementation packages.
• Avoid logic in facade except mapping/forwarding.

Acceptance Criteria

• Existing user imports remain valid.
• Public API signatures remain unchanged.
• Root package contains no heavy implementation logic.
• GO_API_SURFACE.md updated if needed.

[tazarov]

Tracked by umbrella #45 (v0.4.0 root-cleanup plan).

[tazarov]

Completed: Root package uses type aliases (type Server = runtime.Server) and thin wrapper functions. Zero implementation logic at root. Compile-time API gate in compat_test.go.