refactor: Add type constraints to ActionHandler and ActionProcessor

[takeshishimada]

Summary

Add explicit type constraints to ActionHandler and ActionProcessor to match Feature protocol requirements, ensuring type safety and consistency across the framework.

Problem

Currently, ActionHandler and ActionProcessor lack explicit type constraints:

// Current (missing constraints)
class ActionHandler<Action, State, ActionResult: Sendable>

// Feature protocol (has constraints)
protocol Feature: Sendable {
  associatedtype Action: Sendable     ✓
  associatedtype State: AnyObject     ✓
  associatedtype ActionResult: Sendable ✓
}

This inconsistency:

• Weakens type safety when using ActionHandler directly
• Doesn't prevent common errors at compile time
• Creates confusion about requirements

Changes

ActionHandler.swift

• ✅ Add Action: Sendable constraint
• ✅ Add State: AnyObject constraint
• ✅ Add documentation explaining each constraint's purpose

ActionProcessor.swift

• ✅ Add same constraints to ActionProcessor class
• ✅ Add constraints to ActionExecution typealias
• ✅ Add documentation explaining type safety guarantees

Type Constraints Added

public final class ActionHandler<
  Action: Sendable,        // ← NEW: Safe concurrent action dispatching
  State: AnyObject,         // ← NEW: Reference semantics for mutation
  ActionResult: Sendable
>

public typealias ActionExecution<
  Action: Sendable,         // ← NEW
  State: AnyObject,          // ← NEW
  ActionResult: Sendable
> = @MainActor (Action, State) async -> ActionTask<...>

Benefits

1. Prevents value-type State errors

   // ❌ Now caught at compile time
   struct State { var count: Int }  // Error: State must be AnyObject
   
   // ✅ Correct
   @Observable final class State { var count: Int }

2. Prevents non-Sendable Action data races

   // ❌ Now caught at compile time
   enum Action {
     case update(NonSendableData)  // Error: Action must be Sendable
   }

3. Ensures consistency with Feature protocol

   • Same constraints everywhere
   • Clear type safety guarantees
   • Better developer experience

Impact

• Non-breaking change ✅
  • All existing code satisfies these constraints through Feature protocol
  • Tests remain unchanged and pass
• Compile-time safety improvements ✅
  • Catches errors earlier in development
  • Better type inference
• Documentation improvements ✅
  • Clear explanation of constraints

Test Results

• ✅ All 255 tests pass
• ✅ Build succeeds (1.17s)
• ✅ SwiftLint clean
• ✅ No behavior changes

🤖 Generated with Claude Code