Introduces HushChatHeaderType::Control — a self-addressed control signal
(delete / restore / mark-sensitive) that rides a chat tx as a header memo
carrying an `op`, paired with a plaintext label payload so it groups like a
normal header/payload pair. The op VALUE is not restricted at parse time so a
future op reaches newer clients and is ignored by older ones; unknown header
types still fail-closed (grouped as an invalid header, never surfaced, never a
crash), preserving forward-compat.
Foundation for the on-chain chat delete/restore feature (Phase A).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>