Add 2.x → 3.0 migration guide

[liquidsec]

Summary

• New docs/migration/3.0_breaking_changes.md covering CLI/targeting, flags, presets, modules (incl. the http name reassignment), events (VULNERABILITY removal, dict-backed URLs, no confidence attr), config renames, DNS/HTTP engine rewrite, Poetry→uv, Python ≥3.10
• Added a Migration section to mkdocs.yml and the README docs TOC
• Added an upgrade callout near the top of the README pointing at the guide

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90%. Comparing base (ef65b09) to head (cdc2771).
⚠️ Report is 69 commits behind head on dev.

Additional details and impacted files

@@          Coverage Diff           @@
##             dev   #3128    +/-   ##
======================================
+ Coverage     90%     90%    +1%     
======================================
  Files        453     452     -1     
  Lines      46101   46622   +521     
======================================
+ Hits       41211   41720   +509     
- Misses      4890    4902    +12     

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