Optimize go-to-line using buffer / index

[Cameron7195]

Summary
Speed up navigation to arbitrary lines (e.g., line 2,000,000) in large JSONL files via efficient file buffering / indexing.

Problem
“Go to line” gets noticeably slower as the target line increases, suggesting a naive line-by-line traversal. This makes working with very large JSONL datasets (hundreds of thousands to millions of lines) painful.

Proposed Solution

• Implement a more efficient strategy, e.g.:
  • Build and cache a sparse line index (byte offsets for every Nth line).
  • Use file pointers / streams to seek near the desired line, then scan only locally.
• Ensure the index is built lazily and incrementally so initial open is still fast.
• Consider memory usage trade-offs for gigantic files.

Acceptance Criteria

• Measurably reduced latency for:
  • Jumping to line 5,000, 50,000, and 500,000+ in a large JSONL test file.
• Go-to-line operation feels near-constant time for common dataset sizes used in practice.
• No excessive memory usage or blocking UI while building/using the index.

[Cameron7195]

is binary search a solution here? or unnecessary