fix(formatter): preserve by and semicolon for bare genai ability declarations

[Developer-Linus]

Summary

Fixes a formatter bug where bare genai ability declarations like
def foo(...) -> T by <name>; lost the by keyword and trailing ;.

Root Cause

DocIRGenPass.exit_name only rendered the name text and ignored child tokens (KW_BY, Semi) attached to the Name node.

Fix

Update exit_name to detect KW_BY in nd.kid, include the by keyword, and preserve the trailing semicolon:

by <name>;

Test

Adds a regression test to ensure:

• by is preserved
• Output ends with ;
• by llm; appears correctly
• Formatter is idempotent

Files Changed

• doc_ir_gen_pass.impl.jac — fix exit_name
• test_jac_format_pass.jac — add regression test