ETF Fair Value Calculator

Design a class ETF that can track the fair value of a portfolio of holdings. Each holding is a stock identified by a symbol and has two attributes: number of shares and price per share. The ETF class must expose three public methods:

• add_holding(symbol, shares, price) – add or update a holding. If the symbol already exists, accumulate the shares (do not overwrite) and update the price to the new value.
• update_price(symbol, price) – change the price of an existing holding. If the symbol does not exist, the call is a no-op.
• get_fair_value() – return the current fair value of the ETF, defined as the sum over all holdings of (shares × price). The result must be rounded to two decimal places. You may assume that symbols are strings, shares are non-negative integers, and prices are positive decimals. The internal representation is up to you, but the operations must be efficient under heavy usage.