declare fullAccess as constant to reduce gas and improve clarity (issue: #294)

[KimH4nKyul]

Code changes have been made for the reported issue.

Changes:

// Before
uint256 private fullAccess = 3;

// After  
uint256 private constant fullAccess = 3;

Impact of changes:

• Gas optimization: Saves 97 gas per variable access (SLOAD → PUSH operation)
• Storage efficiency: Eliminates unnecessary storage slot, reducing deployment costs
• Code stability: Prevents accidental value modification through constant declaration

Hope this helps.