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

[chaliy]

Summary

• Add MAX_GETLINE_CACHED_FILES (100) limit on distinct files cached by AWK getline var < file
• Enforce per-file size limit from FsLimits::max_file_size in ensure_file_loaded()
• Add 3 tests: cache limit exceeded, within limit, and file size limit

Test plan

• test_awk_getline_file_cache_limit_exceeded — verifies files beyond limit are rejected
• test_awk_getline_file_cache_within_limit — verifies normal usage works
• test_awk_getline_file_size_limit — verifies oversized files are rejected
• All 80 existing AWK tests pass

Closes #988