Add opaque-digest handoff bodies with corrected discriminators

[germ-mark]

createNewAgentHandoff and PublicAnchorAgent.verify(anchorHandoff:…) gain overloads taking groupContext/mlsUpdateDigest as Data, backed by v2 signature bodies that commit to those bytes as length-framed values and never interpret them. The motivation is ownership: a digest's algorithm is a facet of the MLS backend's cipher suite, so requiring a TypedDigest meant the backend could not adopt a new suite until this package shipped a matching DigestTypes case — a release dependency pointing the wrong way. This is safe because a verifier never parses a digest out of a handoff; it rebuilds the body from its own locally derived reference digest and checks the signature against that, so agreement on the algorithm is enforced where the session is established rather than here.

Purely additive. The typed overloads and the v1 bodies are untouched, so every already-issued handoff keeps verifying and nothing needs to re-pair — which is why this is v2-alongside rather than a replacement: classical relationships are live.

The discriminator fix you were looking for. v1's ActiveAgentBody and RetiredAgentBody carry each other's names. It's a labeling bug rather than a security one — domain separation needs the committed strings to be distinct, not correctly named, and they are, so nothing can be replayed across contexts. But it's a real landmine for anyone writing a spec or a second implementation from the names. It is now frozen and commented (renaming in place would silently fail verification for every live relationship), and v2 carries the corrected names. They're .v2-suffixed because the plain corrected strings are unavailable — v1 has them live on the opposite structs, so reusing one would put two different structures under a single committed string. ActiveAnchorBodyV2 takes the suffix too, though its name was never swapped, to keep discriminator↔encoding strictly 1:1.

Tests: 6 new, full suite 118 green. The round trip runs on a digest tagged 0x7F — a value TypedDigest cannot represent at all — which is the property the change exists for. v1↔v2 non-verification is pinned in both directions, and one test asserts all six discriminators are pairwise distinct while another pins the frozen swap so nobody "corrects" it later.

Adoption is not urgent: the app can keep calling the typed overloads by lifting the backend's bytes with TypedDigest(wireFormat:), which is byte-identical. Moving to these overloads is what removes the last cross-repo release coupling, and it costs a PQ re-pairing whenever you choose to do it.

[changeset-bot]

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Latest commit: b3842d1

The changes in this PR will be included in the next version bump.

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@germ-network/autonomous-comm-protocol	Minor

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