Feature: Improve adjacent text highlight UX

[cuspymd]

Summary

Highlighting adjacent text ranges has a high failure rate and is difficult to control.

Problem

• Boundary handling is unstable when selecting contiguous ranges
• Fine-grained adjustment of highlight ranges is difficult, leading to frequent retries

Request

• Improve correction logic for adjacent-range selection
• Improve include/exclude handling for boundary characters
• Improve retry UX (visibility/feedback) when highlighting fails

Expected Outcome

• Higher success rate for highlighting adjacent ranges
• Less stress during precision editing workflows