`common/json.go` and `relaykit/relayconvert/kitutil/json.go` were two
hard-wired copies of the same encoding/json wrapper, so swapping the JSON
engine required editing both modules.
- kitutil defines a `Codec` interface with a standard-library default and
a `SetCodec` hook, mirroring the existing SetLogging host hook; every
kitutil JSON helper and relaykit DTO (un)marshal method goes through it
- `common/json.go` forwards to kitutil and injects `hostJSONCodec` from
init() so tests run on the same engine as production; swapping the
engine now touches only this type in the root module
- route the remaining direct encoding/json calls inside relaykit
(dto/values.go, responses stream validation) through kitutil
- add a codec routing test and a host codec conformance test locking the
encoding semantics the DTOs depend on
Direct encoding/json call sites in the root module are left for a
separate cleanup.
