refactor: extract definition_start helper for description-aware spans

[Boshen]

Ten definition parsers repeated the same two-line expression to pick a span start: the description's start if one was parsed, otherwise the current token. Name it once as Parser::definition_start.

No behavior change. Validated with cargo fmt --check, cargo clippy --all-targets, and cargo test.

[codspeed-hq]

Merging this PR will not alter performance

✅ 5 untouched benchmarks
⏩ 5 skipped benchmarks¹

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_{Comparing refactor-description-start (07f0b3f) with main (822314b)}

Footnotes

1. 5 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩