addPiece

Store the bytes of one metadata piece. Mirrors the validity checks in received_metadata:

• piece must be in range,

• the piece must not overflow the total ((piece*16K + len > total) is rejected),

• and a piece other than the last must be exactly UtMetadata.PIECE_SIZE long (libtorrent enforces this implicitly because it sizes the buffer from total_size).

Re-adding an already-present piece is a no-op that still returns true.