[WIP] Add difflib module for text comparison and diffing

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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This section details on the original issue you should resolve

<issue_title>feat(stdlib): add difflib module — text comparison and diffing</issue_title>
<issue_description>## Summary
Add a difflib module for computing text differences, matching Python's difflib module API. Custom implementation using the Ratcliff/Obershelp pattern matching algorithm.

Proposed Sharpy API

import difflib

diff: list[str] = difflib.unified_diff(a_lines, b_lines, fromfile="a.txt", tofile="b.txt")
diff: list[str] = difflib.context_diff(a_lines, b_lines)
diff: list[str] = difflib.ndiff(a_lines, b_lines)

sm = difflib.SequenceMatcher(None, a, b)
ratio: float = sm.ratio()
blocks: list = sm.get_matching_blocks()
opcodes: list = sm.get_opcodes()

matches: list[str] = difflib.get_close_matches("appel", ["ape", "apple", "peach"], n=2, cutoff=0.6)

Key Decisions

• SequenceMatcher implements Ratcliff/Obershelp algorithm (matching Python)
• get_close_matches for fuzzy string matching (useful for "did you mean?" suggestions)
• HtmlDiff for visual HTML diff tables
• Autojunk heuristic for handling large sequences

Test Plan

• unified_diff and context_diff output format
• ndiff output
• SequenceMatcher ratio/get_matching_blocks/get_opcodes
• get_close_matches fuzzy matching
• HtmlDiff table generation
• Empty sequence edge cases
• Large input performance
  </issue_description>

Comments on the Issue (you are @copilot in this section)

[antonsynd]

Superseded by batch 9 implementation on feat/stdlib-batch9 branch (commit 7668dc9). PR was empty/WIP — difflib implemented from scratch with SequenceMatcher, Differ, unified_diff, context_diff, ndiff, get_close_matches, restore.