fix: Preserve ToC labels for stashed headings

[gaoflow]

Summary:

• Preserve ToC labels for duplicated headings whose visible text contains stashed inline HTML.
• Avoid treating heading placeholders as markdown-exec output placeholders.
• Add a regression test covering a markdown-exec source="material-block" block on the same page as mkdocstrings headings with inline code.

Tests:

• uv run pytest -q
• uv run ruff check src/markdown_exec/_internal/processors.py tests/test_headings.py
• uv run ruff format --check src/markdown_exec/_internal/processors.py tests/test_headings.py
• uv run python -m compileall -q src/markdown_exec/_internal/processors.py tests/test_headings.py
• git diff --check

Issue #97: #97

[pawamoy]

Thanks for the PR @gaoflow, however you didn't respect our PR template, so I'm gonna close. Parsing HTML is a no-go anyway.