fix(builtins): limit AWK getline file cache to prevent memory exhaustion

[chaliy]

Summary

• Add MAX_GETLINE_CACHED_FILES constant (100) to cap files cached by getline var < file
• Guard in ensure_file_loaded() rejects new files when limit reached
• Prevents memory exhaustion from opening hundreds of large files via getline

Test plan

• Opening 105 files via getline — only 100 succeed
• Opening 10 files — all succeed
• File size limit enforcement via getline
• All existing AWK tests pass

Closes #988