[ac1982]

Reusing an HTML2Text instance can add a leading space to emphasized text in later documents. For example, calling handle("<em>hello</em>") twice produces _hello_ first and _hello_ second. Text from a previous plain document can also affect a later emphasized document.

Clear preceding_data at the start of handle, alongside the existing start reset, so spacing decisions only inspect the current document. Add repeated-call cases across all eight emphasis/strike tags and a cross-document case. Update the changelog and contributor list as requested by the contribution guide.

Related to #375. #362 resets start for repeated calls, but leaves the text used by emphasis spacing intact.

Validation on Linux/arm64:

• Nine new cases fail before the fix.
• Full suite: 221 passed on Python 3.9.25 and 3.13.16 (three pre-existing invalid-escape warnings on 3.13).
• Black, Flake8, isort, and mypy --strict html2text pass.

AI assistance: this change and its tests were developed and verified with an AI coding agent.