Function planMailboxPut

planMailboxPut(
    viewSecret: Uint8Array,
    counter: number,
    envelope: Uint8Array,
): MailboxLegPlan
Validates and plans a single mailbox put.
Parameters
- viewSecret: Uint8Array
  32-byte secret shared with the recipient
- counter: number
  uint32 rotation counter
- envelope: Uint8Array
  1..2048 opaque envelope bytes
Returns MailboxLegPlan
The leg plan (mailbox id, exact MBR, envelope)
- Defined in src/blockchain/mailbox.ts:255