Token: Implement is_keyword() and is_operator() methods

[pdaxt]

Component

Foundation / Token

Task

Implement helper methods for TokenKind.

Acceptance Criteria

• is_keyword() -> bool
• is_operator() -> bool
• is_literal() -> bool
• precedence() -> Option<u8> for operators
• All methods are const fn where possible

Verify

assert!(TokenKind::Fn.is_keyword());
assert!(TokenKind::Plus.is_operator());
assert!(TokenKind::Int(0).is_literal());

Security Testing (REQUIRED)

• Test all enum variants exhaustively
• Verify no panics on any valid input
• Test edge cases (EOF, Error variants)
• Ensure methods work in const context

Ref: FOUNDATION.md

[pdaxt]

Already implemented in current codebase.