Per inbox membership for app data

[tylerhawkes]

Add GroupMembershipEntry protobuf message for per-inbox membership state

Defines a new GroupMembershipEntry message in group_membership.proto to store per-member inbox state. The message uses a versioned oneof with a V1 variant holding sequence_id and failed_installations fields.

^{Macroscope summarized 07bc44c.}

[macroscopeapp]

Approvability

Verdict: Approved

Purely additive proto message definitions with no modifications to existing types. Adds GroupMembershipEntry with a versioned V1 message structure - backwards-compatible schema addition with no direct runtime impact.

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