Splits ProtocolMessages exceeding NIP-44's 65,535-byte plaintext ceiling
across multiple gift-wrapped events. Symmetric (both directions),
fire-and-forget, backward-compatible via a new transport-level
\`extensions\` field on \`dapp_ready\` and \`wallet_ready\`. Resolves the
\"Failed to swap: invalid plaintext size\" error on aggregated swap
requests and enables signed-tx responses up to the 1 MB BCH consensus
limit (~2 MB hex).
