feat: max position size per trader to limit protocol concentration risk

[abayomicornelius]

Summary

A single whale trader could open a position so large it dominates one side of the OI, making the pool highly directional and increasing the risk of insolvency. A per-trader max position size cap limits this concentration.

Proposed Design

Store max_position_size_usd_key(market, is_long) in data_store (admin-configurable per market side).

On every position increase, check:
current_position_size + size_delta <= max_position_size_usd

Revert with OrderError::MaxPositionSizeExceeded if violated.

A value of 0 means uncapped (default behaviour preserved).

Acceptance Criteria

• Position increase beyond cap reverts with typed error
• Cap of 0 allows any size (no regression)
• Cap is enforced per trader per market per side (not global)
• Integration test: set cap to 50,000 USD, attempt 60,000 USD position — revert
• Integration test: two traders each with 40,000 USD positions — both succeed

Affected Contracts

order_handler, data_store

[Samuel-21-eng]

@Samuel-21-eng has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer, I would love to work on this one

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Samuel-21-eng to this issue.