Header protection (QUIC RFC 9001 §5.4) — the per-packet mask that hides the
variable header fields (packet_number ‖ flags ‖ stream_id ‖ epoch ‖ path_id, the 14 bytes at wire offset [1..15] since ε / WIRE v5; [33..47]
in v4) from a passive on-path observer.
RngProvider — the indirection through which Phantom Protocol obtains
cryptographic randomness. Default is OsRng, which delegates to
getrandom::getrandom and therefore picks up the platform’s CSPRNG on
every supported target (Linux getrandom(2), macOS / iOS
CCRandomGenerateBytes, Windows BCryptGenRandom, wasm32 via the js
feature → crypto.getRandomValues, etc.).