An undeclared key fails the whole reading, so a connector whose ports vary by
model has to narrow what it publishes to what was bound. Names the idiom and
the once-not-per-poll rule for saying a declared port is missing.
Adds reference/connector-contract.md and code/connectors.md to the site, with
the node-type reference finally linking the contract it describes. Strict
build passes.
stroblme