fix(builtins): cap AWK printf width/precision to prevent memory exhaustion

[chaliy]

Summary

• Cap width and precision values to 10,000 in AWK format_string()
• Return error + exit code 2 when limit exceeded
• Apply same cap to bash printf builtin width parsing for consistency
• Prevents ~1GB allocation from printf "%999999999d", 1

Test plan

• awk '{printf "%999999999d", 1}' rejected with error, exit 2
• awk '{printf "%999999999.5f", 1}' rejected with error, exit 2
• awk '{printf "%20d\n", 42}' works correctly
• awk '{printf "%10000d\n", 1}' works at boundary
• All 1929 existing spec tests pass

Closes #983