scientific(membership): bind serialized support evidence to a recomputable wire digest #652

Finding

tepp.membership_observation_support_projection.v1 currently serializes the owner-issued opaque-support support_sha256, but parsed wire validation only checks that this field is a 64-character lowercase hex string. The exact owner digest intentionally hashes raw MemberId/GroupId UUID bytes, while the privacy-reduced projection intentionally omits those identifiers. Therefore a released consumer cannot recompute that owner digest from the serialized support rows, and a byte-valid payload can substitute an unrelated 64-hex support digest while retaining the same reconstructable topology.

This does not make parsed wire authoritative—the parser is already explicitly non-authoritative—but it leaves the released reconstruction bytes without a self-checkable content identity and makes the current “binds the source-support digest” wording too strong for the serialized boundary. #638/#639 need a deterministic consumer coordinate that can be checked from the released bytes without copying Membership source identity.

Required RED

Starting from a valid owner-produced canonical payload:

1. mutate only support_sha256 to another valid lowercase 64-hex value and require parsing to fail unless the wire-content commitment is updated consistently;
2. mutate one reconstructable observation coordinate/assignment while preserving otherwise-valid design/topology and require parsing to fail against the unchanged wire-content commitment;
3. require owner-produced payloads to expose a deterministic versioned wire digest invariant under caller input ordering and network insertion ordering, while duplicate observation multiplicity remains significant.

Minimal causal repair

Keep support_sha256 as the owner-attested opaque-support provenance fingerprint; do not pretend it is recomputable after UUID redaction. Add a separate versioned digest over the canonical privacy-reduced projection payload excluding the digest field itself, and make MembershipObservationSupportWire::from_json recompute and compare it before accepting bytes. The content digest must cover schema/design/support-digest coordinates, counts, canonical event times, local ordinals, roles, exact binary64 weight bits, and support_sha256 so these fields cannot drift independently under the supported parser contract.

Do not add signatures, secrets, source UUIDs, consumer-local hashing, or a second Membership topology classifier. Cryptographic authenticity remains release/provenance responsibility; this issue is deterministic cross-field/content binding at the owner wire boundary.

Refs #605 #638 #639 #646 #649 #650 #651.