Deprecate enveloped-signature after a transform that returns octets #648

Problem

A Reference that applies enveloped-signature after a canonicalization, such as [exc-c14n, enveloped-signature], signs and verifies. XMLDSig §6.6.4 applies that transform only to "a node-set from its parent XML document", so #647 proposes rejecting the order in 7.0. The spec text and the interop measurements are in #647.

Callers need a release that warns before the one that throws, and today nothing tells them the order is wrong:

• The library verifies its own signature in that order, so a round trip passes.
• The README says the reference processing model requires the order to work (README.md#L67-L81).

Proposal

In 6.4:

• Print a DeprecationWarning when enveloped-signature is applied after an earlier transform of the same Reference returned octets, when signing and when verifying. The order keeps signing and verifying, with the same output.
• Change the README to say that enveloped-signature goes before any canonicalization, and that the other order is deprecated and will throw in 7.0.