feat(core): allow omitting context in initial state for optional context types

[Junnis0123]

Summary

• Allow omitting context in useFunnel's initial option when all context properties are optional
• Default TContext to {} in createFunnelSteps()

Changes

@use-funnel/core

• Add FunnelInitialStateByContextMap type (core.ts)

  • Makes context optional when Partial<TContext> extends TContext
  • context remains required when the type has any required properties

• Introduce normalizedInitial (useFunnel.tsx)

  • Normalizes with context ?? {} before passing to router adapters
  • Behavior is identical to the existing code when context is always provided

• Add default type parameter to createFunnelSteps (stepBuilder.ts)

  • createFunnelSteps<TContext>() → createFunnelSteps<TContext = {}>()

Example

// Before: context was always required even if all properties are optional
useFunnel<{ A: { foo?: string }; B: {} }>({
  id: 'test',
  initial: { step: 'A', context: {} },
});

// After: context can be omitted when all properties are optional
useFunnel<{ A: { foo?: string }; B: {} }>({
  id: 'test',
  initial: { step: 'A' },
});