⚡ Optimize LSP boundary character lookups from O(N) to O(1)

[Tcode-Motion]

💡 What: Replaced line.chars().nth() lookup calls in tools/lsp/src/lib.rs with line[..pos].chars().next_back() and line[pos..].chars().next().

🎯 Why: The original code used .chars().nth(byte_offset) inside a loop. This exhibited O(N^2) complexity on long lines because nth() traverses the string from the beginning in O(N) time for every match found. Additionally, because .find() returns a byte offset, passing it to nth() which expects a character index introduced a subtle bug when multi-byte unicode characters were present. The new approach uses the guaranteed-valid byte offset to slice the string and look at the immediately adjacent characters, running in O(1) time.

📊 Measured Improvement: A benchmark was added to verify the impact on a long line containing many matches. The time to execute the boundary checks dropped from ~1.7ms (slow) to ~45µs (fast).

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PR created automatically by Jules for task 10062600667242564767 started by @Tcode-Motion

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