Design a position calculator that tracks net position and totals from trades.

Module name: pos_calc
Parameters:
  - WIDTH = 32       // Data width for positions and quantities

Ports:
  - input  clk, rst_n
  - input  [WIDTH-1:0]        qty          // Trade quantity
  - input                     is_buy       // 1 = buy, 0 = sell
  - input                     trade_valid  // Trade is valid
  - output signed [WIDTH-1:0] net_position // net_position = total_bought - total_sold
  - output [WIDTH-1:0]        total_bought // Cumulative bought quantity
  - output [WIDTH-1:0]        total_sold   // Cumulative sold quantity

Functional specification:
  - On each valid trade: buy adds qty to total_bought, sell adds qty to total_sold
  - net_position = total_bought - total_sold (signed result)
  - Accumulates over time; reset clears all to zero
  - Sequential (registered outputs)
