docs: Improve README based on CoreConcepts documentation

[takeshishimada]

Summary

Enhance the "Result-Returning Actions" section in both English and Japanese READMEs to provide clearer and more detailed explanations, aligning with the comprehensive documentation in CoreConcepts.md.

Changes

README.md (English)

Before:

Actions can return results through `ActionTask`. This enables parent-child 
result passing and allows navigation with a navigator from @Environment, 
keeping everything within the view tree.

[code example]

- Parent-child result passing stays within the view tree
- Navigation possible with navigator from @Environment

After:

Actions can return results through `ActionTask`. The result type 
(`ActionResult`) can be freely defined for each Feature.

In this example, a child view returns a selection result to the parent, 
which handles navigation:

[code example]

This implementation provides:
- `ChildFeature` returns selection results to the parent via `ActionResult`
- `ParentView` receives results through the `onSelect` callback
- Parent controls side effects like navigation
- Everything stays within the view tree, making dependencies easy to track

README_jp.md (Japanese)

Similar improvements applied to the Japanese version with culturally appropriate phrasing.

Improvements

1. Explicit Type Definition: Clarifies that ActionResult can be freely defined per Feature
2. Contextual Introduction: Adds "In this example..." to set up the code sample
3. Structured Benefits: Changes from simple bullets to "This implementation provides:" with detailed points
4. Component Relationships: Explicitly explains how ChildFeature, ParentView, and callbacks interact
5. Dependency Tracking: Emphasizes that everything stays in the view tree for easy dependency tracking

Impact

• ✅ Better understanding of the result-returning pattern
• ✅ Clearer explanation of parent-child communication flow
• ✅ More explicit benefits and architectural advantages
• ✅ Consistent with CoreConcepts.md detailed documentation
• ✅ Improved learning experience for new users

Files Changed

• README.md (+8 lines, -3 lines)
• README_jp.md (+8 lines, -3 lines)

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