Speed up opening boundary matching

[Kludex]

Match complete opening boundaries with bytes.startswith(), retaining byte-by-byte parsing for fragmented or mismatched boundaries and the existing delimiter validation. Keep one regression test for max_size truncating the opening boundary; existing tests cover chunk splitting and malformed boundaries.

Performance

Local CPython 3.13.4 on macOS ARM64, medians of 15 interleaved timeit runs with a fresh parser per iteration:

Two-field form	Before	After	Less parsing time
MultipartParser	22.01 µs	14.39 µs	34.6%
FormParser, collecting fields	30.63 µs	22.72 µs	25.8%

Large uploads and one-byte chunks were effectively unchanged; these are parser timings, not HTTP throughput measurements.

Validation

• 161 tests pass with 100% package coverage.
• Ruff, mypy, check-sdist, and the lockfile check pass.
• The new regression test passes with -W error; the full suite exposes three existing unclosed-file warning failures, reproduced on main.

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-24T19:29:49.421338Z	06d7f18	PR opened

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[codspeed]

Merging this PR will improve performance by 16.71%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 4 untouched benchmarks
⏩ 5 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_parse_multipart_simple_form	498.4 µs	427 µs	+16.71%

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing speed-up-opening-boundary (582240e) with main (fc922ef)}

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. ↩

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}

[cubic-dev-ai]

All reported issues were addressed across 2 files (changes from recent commits).

<sub>Tip: Review your code locally with the cubic CLI to iterate faster.

Re-trigger cubic</sub>