Fix inconsistent arithmetic operations in votor module

[0xahzam]

Problem

The votor module contains inconsistent arithmetic operation patterns.

Specifically:

• Slot arithmetic mixes saturating_add() and checked_add() for identical operations across files
• Timer arithmetic inconsistently uses saturating_ vs checked_ operations
• Multiple .unwrap() calls provide no debugging context when panics occur

Summary of Changes

• Standardize slot arithmetic to consistently use checked_add/checked_sub with overflow detection
• Standardize timer arithmetic to consistently use checked_ operations
• Replace .unwrap() calls with descriptive .expect() messages for better debugging
• Preserve saturating_ operations for stats/stake calculations where overflow clamping is desired

Files modified:

• certificate_pool.rs: 3 slot arithmetic standardizations
• certificate_pool/parent_ready_tracker.rs: 5 slot arithmetic standardizations
• certificate_pool_service.rs: 1 slot arithmetic + error message improvement
• event_handler.rs: 1 slot arithmetic standardization
• skip_timer.rs: 6 timer arithmetic standardizations + error messages

Rationale: Slot and timer overflow indicates consensus logic bugs. Better to crash explicitly with clear error messages than silently corrupt calculations.

Fixes #287

[ksn6]

Really appreciate the contribution here! One minor fix, and we're all set :)

[ksn6]

@0xahzam - once merge conflicts are resolved, this should be good to land in master.

[0xahzam]

@ksn6 perfect! thanks, should I try to resolve the conflict or someone else will pick it up?

[ksn6]

@0xahzam - feel free to take a look at resolving the conflict; these should hopefully be pretty straightforward - don't hesitate to reach out if you run into any issues!

[0xahzam]

@ksn6 I think it should be fine now

[waldrobi]

Is this PR still needed?
Can we close if not