Pricing Engine: Implement volume-weighted pricing strategy

[0xDevNinja]

Objective

Develop a volume-weighted pricing engine that computes a clearing price based on the weighted average of individual order prices. The engine should consider the volume of each order when calculating a uniform clearing price, providing a fairer distribution across large and small orders.

Tasks

• Create a new module src/pricing/volume_weighted.rs that defines VolumeWeightedPricingEngine implementing the PricingEngine trait.
• In compute_clearing_price(&[Order]) -> Option<Decimal>, calculate the weighted average of each order's implied price (buy_amount / sell_amount) using sell_amount as the weight.
• Return None if there are fewer than two orders or if total sell volume is zero.
• Add unit tests that verify:
  • None is returned for zero or one order.
  • The weighted average price is correct for multiple orders with varying sizes.
• Update src/pricing/mod.rs to expose the volume_weighted module.
• Update documentation (README or relevant docs) to describe the new pricing strategy and how to select it via CLI or config.

Acceptance Criteria

• New module compiles without errors and is covered by tests.
• Cargo test passes for all existing and new tests.
• Documentation reflects the new pricing engine and usage.

Closes #? (to be filled when opening PR).