compiler: format statements and complete units

[floitsch]

Part 4 of 10 in the AST-driven Toit pretty-printer stack. Depends on #3188.

Summary

• format declarations, assignments, returns, loops, conditionals, try/finally, and control flow
• format imports, exports, fields, methods, classes, and complete source units
• preserve declaration order and emit one final newline
• fail explicitly on unsupported AST kinds

This completes the comment-free AST printer before comment ownership is introduced.

Verification

• statement and unit formatting builds independently
• complete SDK units are exercised by the corpus gate at the top of the stack

_{Stack created with GitHub Stacks CLI}