Add revised TIP-796: Native Username Standard for TRON Accounts

[alichatme]

Summary

This pull request contains a clean, revised version of TIP-796.

The proposal has been updated based on the feedback received during the previous discussion. The revision includes improvements to the specification, clearer definitions, and additional explanations for the username assignment and verification workflow.

Major updates include:

• Refined staking-based username assignment model
• Username retention and reassignment protection periods
• Anti-abuse and anti-squatting mechanisms
• Username structure and assignment rules
• Transaction verification workflow
• Security benefits and threat model
• Existing account integration
• Improved rationale and overall document clarity

This PR supersedes the previous draft and is submitted as a clean revision following the review feedback.

Thank you for your time and review.

Discussion
Related Issue: #799