docs: Clarify actor execution order guarantees in Store

[takeshishimada]

Summary

Fix misleading documentation that claimed actions are processed "sequentially" in Store. Per SE-0306, Swift actors (including MainActor) do NOT guarantee FIFO ordering - the runtime uses priority-based scheduling.

Problem

The current documentation states:

Actions are processed sequentially on the MainActor.

This is misleading. While actions execute on MainActor (ensuring thread-safety), multiple send() calls without await may execute in any order due to Swift's priority-based task scheduling.

Changes

• ✅ Replace "Sequential Processing" section with accurate "MainActor Execution and Task Ordering"
• ✅ Add explicit reference to SE-0306 with direct quote from the proposal
• ✅ Add Warning about non-guaranteed execution order
• ✅ Provide correct usage examples for guaranteed sequential execution using await
• ✅ Add implementation comments explaining task ordering behavior
• ✅ Update processAction() documentation for clarity

Technical Details

SE-0306 Quote:

"Tasks awaiting an actor are not guaranteed to be run in the same order they originally awaited that actor."

Before:

store.send(.action1)
store.send(.action2)  // Will wait for action1? ❌ NO!

After (Correct Documentation):

// ⚠️ Order NOT guaranteed
store.send(.action1)
store.send(.action2)

// ✅ Guaranteed sequential execution
await store.send(.action1).value
await store.send(.action2).value

Impact

• No behavioral changes - only documentation/comments
• All tests pass (255 tests)
• SwiftLint clean
• Helps developers understand actual runtime behavior and use correct patterns

Test Plan

• Build succeeds
• All unit tests pass
• SwiftLint validation passes
• Documentation is technically accurate per SE-0306

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