Mistune 3 parser refactor + CI tooling

[philippb]

Goal

Upgrade to the Mistune 3 parser stack while preserving master behavior for structure diff output.

What Changed

• Refactored parser internals for Mistune 3 token APIs while keeping public sdiff behavior stable.
• Fixed inline token buffering regression (links/images/markers/linebreaks) that could duplicate rendered text.
• Restored legacy-compatible parsing semantics for key markdown edge cases.
• Added/expanded parser and compatibility-focused tests.
• Aligned lint configuration with branch CI expectations.
• Added coverage-focused tests to satisfy repository coverage gate.

Validation

• make test passes on refactor/mistune3-api.
• python -m flake8 --config .flake8 sdiff tests passes.
• python -m coverage report -m passes with total coverage above fail_under=96.

Compatibility Notes

• Objective is behavioral compatibility with master for structure diff, not markdown auto-formatting.
• Any behavior differences intended as improvements should be called out explicitly before expectation changes.