Disallow 'bool' type in .trb files - enforce 'Boolean' only

[yhk1038]

Summary

Currently, both bool and Boolean are accepted in .trb source files. For consistency, we should only allow Boolean in user-facing .trb files.

Current Behavior

# Both work currently
def is_valid(): bool    # Should error
def is_valid(): Boolean # Correct

Expected Behavior

• .trb files: Only Boolean should be valid
• .rbs output: Compiler converts Boolean → bool (RBS standard)

Context

• Boolean is defined in T-Ruby stdlib as type Boolean = true | false
• RBS standard uses lowercase bool
• TypeScript uses lowercase boolean, but T-Ruby follows Ruby naming conventions (PascalCase for types)

Implementation Options

1. Parser level: Reject bool during type annotation parsing
2. Type checker level: Add bool to list of unknown/invalid types

Related

• Documentation already uses Boolean consistently
• Test code updated to use Boolean in .trb examples