refactor(CampaignInstance): consolidate state variables into structs to resolve linter violations and stack depth issues

BREAKING CHANGE: State variable access patterns updated from direct access to struct member access

- Group 24 state variables into 3 logical structs (CampaignCore, DonationConstraints, Counters)
- Reduce state variable count from 24 to 11, resolving max-states-count linter violation
- Eliminate stack too deep compilation errors by reducing parameter passing overhead
- Remove redundant milestoneCount variable; derive from counters.nextMilestoneId
- Update all function bodies to use struct member access (e.g., core.creator instead of getCreator)
- Add interface compliance getter functions to maintain external API compatibility
- Implement pull-based refund pattern with claimRefund() and batchRefund() functions
- Fix type mismatch errors in comparison operations (totalRaised vs goalAmount, state vs CampaignState)
- Optimize gas consumption by eliminating duplicate state writes
- Improve code organization and maintainability through logical data grouping

Affected functions: initializeCampaign, createMilestone, finalizeCampaign, donate,
withdrawMilestoneFunds, pauseCampaign, resumeCampaign, cancelCampaign, and 40+ others

Fixes: #[issue-number] - Stack too deep compiler error
Fixes: #[issue-number] - Linter max-states-count violation (24 > 20 limit)

Author: olujimiAdebakin