fix(contracts): check virtual offset additions

[samlogy1]

closes #569

Return ContractError::Overflow when pricing offset additions exceed i128 instead of panicking.

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Summary

Test plan

• pnpm lint && pnpm typecheck && pnpm test pass locally
• [ ]

Closes #

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[collinsezedike]

@samlogy1 the fix itself is correct, both additions now go through checked_add before the multiply/divide, matching the issue's proposed solution exactly. Two things needed before this can merge:

• No test covers the overflow path this fix adds. Add one that drives total_shares/total_assets toward the limit where the addition would overflow and asserts ContractError::Overflow is returned. PR #605 in this repo is a good model for how to trigger an overflow in a test.
• The PR body is still the unfilled template: ## Summary has no bullet, both ## Test plan boxes are unchecked, and there's a dangling Closes # with no number at the end on top of the closes #569 already at the top. Please fill it in per CONTRIBUTING.md's PR body format.