Refactor balance and supply math to use checked arithmetic helpers

[Ibinola]

Description

Balance and supply updates use direct + and - operations today. Even with overflow checks enabled in release, the code should centralize arithmetic through checked helpers so failure modes are explicit and reusable across modules.

Acceptance Criteria

• Balance and supply math uses shared checked helpers rather than raw arithmetic
• Overflow/underflow failure behavior is deterministic and tested
• Escrow/splitter/recurring logic reuses the same checked math primitives where relevant

Technical Scope

• Files likely affected: src/balance.rs, src/contract.rs, src/escrow.rs, src/splitter.rs
• Modules/contracts involved: balance, VeritixToken, escrow, splitter

Dependencies

• Enforce positive amount validation across all balance-moving entrypoints

Notes

If the team prefers panic-based failures over custom error enums, keep the helper surface small and consistent.