feat: rust-style beautiful error messages

[cs01]

Summary

• Compile errors now display Rust-style formatting with source code context, line numbers, colored underlines pointing to the exact token, and help/note annotations
• Before: error: class 'User' has uninitialized field 'name' — assign a value...
• After:

error: class 'User' has uninitialized field 'name'
   --> file.ts:2:3
   |
 2 |   name: string;
   |   ^~~~
   |
   = help: assign a value in the field declaration or constructor

• All 4 semantic passes (closure mutation, union types, type assertions, uninitialized fields) now use the shared formatCompileError() formatter
• Fixed a bug where entryFileCode wasn't found for relative input paths (abs path comparison mismatch)
• Uninitialized field errors now point to the field declaration line, not the class line

Test plan

• All 458 tests pass
• Self-hosting passes (Stage 0 + Stage 1)
• Native compiler builds successfully
• Compile error test fixtures still match expected substrings

🤖 Generated with Claude Code