Tracks follow-up work from #66 after the copied misbehaving-flow-control fixtures were promoted.\n\nCurrent limitation:\n- raw DATA handling uses a cumulative lifetime byte counter\n- there are no independent connection and stream receive windows\n- consumed data does not restore credit through WINDOW_UPDATE\n- incoming zero or overflowing WINDOW_UPDATE frames are not validated\n\nAcceptance criteria:\n- debit connection and stream receive windows for every DATA frame\n- restore credit only when buffered bytes are consumed according to stream flow state\n- serialize and process WINDOW_UPDATE correctly\n- reject zero increments and overflow with protocol-correct errors and ordering\n- add positive coverage where a flowing stream transfers more than initialWindowSize cumulatively\n- retain negative coverage for paused or burst senders that exceed available credit\n- keep the relevant Bun and Node HTTP/2 corpus tests green\n\nParent tracker: #66