Fold internalbridge into the root package

internal/internalbridge was a pure pass-through layer: it mirrored the public request types field-for-field, wrapped syncer.Config in an opaque struct, and forwarded Probe/Run calls verbatim. The root client had to convert every request through bridge mirror types, and unstable/client.go round-tripped through them just to build a syncer.Endpoint literal.

Define the public result types (RefResult, Stats, ProbeResult, SyncResult, ...) directly in the root package instead of aliasing internal types -- this also makes their godoc visible on pkg.go.dev and exports SideBytes, which was previously reachable through Stats.Sides but not nameable by callers. Build syncer.Config directly in the client, matching what unstable/client.go already does. Field sets and JSON tags are unchanged, so the published API surface is identical.