feat: Added @perseveranza-pets/milo as alternative experimental HTTP parser

[ShogunPanda]

Summary

• Added an opt-in Milo-backed HTTP/1 parser, enabled with UNDICI_USE_MILO=1 or UNDICI_USE_MILO=true.
• Vendored Milo CJS artifacts, including SIMD and non-SIMD WebAssembly builds, plus a build:milo script to refresh them.
• Integrated Milo into the HTTP/1 client parser selection while preserving the existing llhttp parser as the default.
• Updated benchmarks and tests for the new parser callback flow and Milo’s stricter RFC 9112 parsing behavior.

Assisted-By: OpenAI:GPT-5.5 <openai/gpt-5.5>

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• [S] Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[ronag]

Perf comparison?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 12.30509% with 4668 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.27%. Comparing base (838b6cd) to head (6bfe2bd).

Files with missing lines	Patch %	Lines
lib/milo/src/no-simd/index.js	0.00%	1087 Missing ⚠️
lib/milo/src/no-simd/unbundled.js	0.00%	1085 Missing ⚠️
lib/milo/src/simd/unbundled.js	0.00%	1085 Missing ⚠️
lib/dispatcher/parser-h1.js	13.36%	810 Missing ⚠️
lib/milo/src/simd/index.js	44.80%	600 Missing ⚠️
lib/dispatcher/client-h1.js	97.72%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5492       +/-   ##
===========================================
- Coverage   93.43%   83.27%   -10.16%     
===========================================
  Files         110      115        +5     
  Lines       37278    42536     +5258     
===========================================
+ Hits        34829    35421      +592     
- Misses       2449     7115     +4666     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[ShogunPanda]

Perf comparison?

I haven't posted them here but even thought Milo is twice as fast as llhttp in terms of pure WASM perf, undici benchmark varies from +3% to -1% on different runs. I've tried countless times.
The interesting thing is that Milo has an optimization which can suspend the parser after the headers and I have this in the H1Parser so body parsing can be skipped when the Content-Length is present. This would help performance in big payloads.

[metcoder95]

I like this, a env var seems a good option as well to switch between experiences; but maybe adding it within a Client constructor can be helpful.

[ShogunPanda]

I like this, a env var seems a good option as well to switch between experiences; but maybe adding it within a Client constructor can be helpful.

Do you mind to expand. I'm not sure I get what you're asking for.