Adds sender_pubkey + sender_seq fields to MeshMessage so received
messages carry a stable cross-transport identity: (sender_pubkey,
sender_seq) pair. This is the foundation for the upcoming reply,
reaction, edit, and read-receipt variants — they need to target a
message by an ID that is meaningful on every node, not just locally.
Receive-side population lives in dispatch.rs::store_typed_message,
which now looks up the peer's pubkey_hex and copies envelope.seq from
the decoded TypedEnvelope. Sent-side population will land when we
plumb a per-node monotonic seq counter through the RPC layer.
Also adds mesh.debug-dump: a full in-memory state snapshot returning
peers, messages, status, shared-secret peer ids, encrypt_relay flag,
and stego mode — intended for smoke tests and bug investigation.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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