nx_tls13_app_send_fd_gate.nx

nx_tls13_app_send_fd_gate.nx -- ENGINEER gate for the chunked TLS app-data sender. Evidence-driven, re-runnable, no network: fabricates a CONNECTED Tls13ServerSession with fixed ChaCha20-Poly1305 keys, sends a 40000-byte payload to a real file fd, then plays the CLIENT side -- walks the wire bytes record by record, asserts every record plaintext <= 16384 (the RFC 8446 §5.1 cap the old single-record path violated), decrypts each record with the KAT'd nx_tls13_record_decrypt_v2 under the same keys, reassembles, and byte-compares against the original payload. Also gates the empty- payload path (one empty record) and the partner invariant: exactly ceil(40000/16000) = 3 records, seq increments 0,1,2. license_tier: ORIGINAL