refactor: consolidate initial value derivation logic

[michaelbe812]

Summary

• Consolidated duplicate initial value derivation logic into a single utility function
• Eliminated code duplication while maintaining exact same behavior

Problem

The codebase had two separate implementations for determining initial values during refresh operations:

1. calcStartValueForRefresh utility function (with tests but unused)
2. deriveInitialValue inline function in create-state.ts (no direct tests)

This duplication created maintenance burden and risk of inconsistent behavior.

Solution

• Refactored deriveInitialValue to internally use the existing calcStartValueForRefresh utility
• Kept both functions to maintain API compatibility
• deriveInitialValue is now a thin wrapper that creates an RxJS operator using the centralized logic

Changes

• Added import for calcStartValueForRefresh in create-state.ts
• Simplified deriveInitialValue to delegate to calcStartValueForRefresh
• Removed 19 lines of duplicate code

Testing

• ✅ All existing tests pass
• ✅ Linting passes
• ✅ No behavioral changes - the refactored code produces identical results

Benefits

• Single source of truth for initial value logic
• Better test coverage (tests already exist for calcStartValueForRefresh)
• Easier maintenance
• Clear separation of concerns

Fixes #51