Decentralized Fork Recovery Updates

[codabrink]

Add MessageType enum and ForkAdminChange message to MLS protocol for fork recovery

• Deprecates the is_commit boolean in GroupMessage.V1 and ValidateGroupMessagesResponse.ValidationResponse, replacing it with a message_type field using a new MessageType enum that includes APPLICATION, COMMIT, and FORK_ADMIN_CHANGE values.
• Adds a new ForkAdminChange wire message in commit_log.proto with sequence ID, HMAC, signature, and group ID fields to support replay-protected fork admin operations.
• Updates CommitLogEntry to carry a new installation credential signature (field 4) over the commit payload and group salt; the previous signature field is deprecated but retained for backward compatibility.
• Behavioral Change: Consumers of GroupMessage.V1 and validation responses should migrate from is_commit to message_type; the deprecated fields remain on the wire but carry no new information.

^{Macroscope summarized 16d3ffc.}

[macroscopeapp]

Approvability

Verdict: Approved

Backward-compatible protobuf schema additions (new enum, new fields, deprecations) for fork recovery functionality. Author is an admin who owns all changed files. Changes are additive with no breaking changes.

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Dismissing prior approval to re-evaluate a4c9c89

Dismissing prior approval to re-evaluate 16d3ffc