Generalize async eval and replace

[nooga]

Summary

• make C-c C-v a universal asynchronous eval-and-replace command in let-go mode
• add structural form detection, evaluation highlighting, stale-source protection, and swappable evaluator/result backends
• let vibe supply its own context and tagged source-aware result without owning the key binding
• document the path toward an nREPL backend
• preserve empty newline pieces during insertion so RET works with current let-go
• suppress core-symbol replacement warnings during normal startup

Testing

• Full suite: 373 tests, 1,673 assertions
• 0 failures, 0 errors
• lg main.lg loads without namespace replacement warnings