the engine declared size matchers and kv extractors but the executor
dropped them (size fell through to the default case, kv was never read).
wire both: size matches the response body length in bytes, kv records
every response header as a key-value pair namespaced by the extractor
name.
this unblocks the headers.go conversion in #52, which needs a full header
dump the known-set regex extractors cannot reproduce; the headers.yaml
module and the headers.go removal are a separate follow-up. the extractor
is named kv to match docs/modules.md (the struct comment said kval). the
declared json extractor stays deferred since it needs a json-path
dependency and a path-syntax decision.
refs #52
Tigah