Introduce the canonical lnwire.BlindedPath / BlindedPaths codec with a
sealed IntroductionNode sum-type covering both the BOLT 4 pubkey and
sciddir variants. The codec gates every variable-length subfield against
an io.LimitedReader. It fails closed on the encoder side so invalid
input never hits the wire.
This commit is a pure addition: no existing caller changes. Subsequent
commits migrate OnionMessagePayload and the bolt12 message structs to
consume the new codec.
