feat: Implement intelligent sequence matching for frame offsets in test comparison

[DYHARDx]

Description

This PR addresses a core requirement mentioned in the "Sample Platform NG" GSoC brief: improving the regression test comparison algorithm to gracefully handle frame offsets.

Previously, mod_test/nicediff/diff.py evaluated the actual vs. expected test results strictly line-by-line (1-to-1 index matching). If a single subtitle frame was dropped or offset, it triggered a chain reaction where all subsequent frames were incorrectly labeled as a mismatch, causing the entire test to fail unnecessarily.

This PR refactors get_html_diff to leverage Python's built-in difflib.SequenceMatcher.

Changes Made

• Added import difflib
• Replaced the naive for loop matched-index comparison in get_html_diff with sm.get_opcodes().
• The logic now properly identifies opcodes (replace, insert, delete, equal) and parses the text into HTML blocks.
• Missing frames are explicitly rendered as deletions or insertions (diff-table-td empty rows mapping), ensuring the rest of the output resyncs and reads accurately without a false-negative cascading failure!

Related Issue

Fixes #1079

GSoC Context

I am submitting this PR as my qualification task for GSoC (Sample Platform NG project).

Checklist before review

• I have read and understood the contributors guide.
• I have verified the changes locally
• Code passes the PEP8/Pycodestyle styling checks (pycodestyle ./ --config=./.pycodestylerc)

[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud