Consider making --at-once the default processing mode

[jlevy]

Summary

The current default is line-by-line processing, requiring --at-once for multi-line regex patterns. There is a TODO at line 1639 asking whether --at-once should be the default.

Discussion

Arguments for --at-once as default:

• Multi-line patterns are a common use case (matching across line boundaries)
• --dotall is useless without --at-once, which is a confusing interaction
• Modern systems have abundant memory; the "files might be too large" concern is rarely relevant
• Users who want multi-line matching must remember to add --at-once, which is a common source of confusion

Arguments for keeping line-by-line as default:

• Predictable memory usage for very large files
• Preserves backward compatibility
• Line-by-line is the traditional behavior (matches sed conventions)

Suggestion

Consider either:

1. Making --at-once the default with a --line-by-line flag for the old behavior
2. At minimum, adding a warning when --dotall is used without --at-once (since the combination is almost certainly a mistake)

Option 2 is lower-risk and addresses the most common confusion.