http: add native single-shot response builder

[anonrig]

Summary

Add a C++ buildHttpMessage helper used by the HTTP/1.1 outgoing path so the common cases avoid repeated JavaScript string concatenation and multi-write overhead:

• _storeHeader: builds the status/request line + header block natively (with JS fallback for special cases such as content-disposition latin1 / unique cookie joining).
• ServerResponse.end() fast paths:
  • Headers not yet rendered: full message (headers + body) built in C++ and written once.
  • After writeHead(): tryFastFlushEnd flushes the prebuilt header + body without the full write_() / extra empty-write path (this is the benchmark/http/simple.js case).
• Server-side native headers stay as Buffers until write time (no latin1 string copy on the hot path). ClientRequest still materializes a string for _header compatibility.
• Complex encodings (hex, utf16le, ...), trailers, and fake/standalone sockets keep the existing JS path.

Public API and on-the-wire format are unchanged.

Benchmarks

Machine-local out/Release on the same host. Baseline measured before this change.

benchmark/http/simple.js (primary)

type=bytes len=4 chunks=1 c=50 chunkedEnc=0 duration=5

	RPS
Baseline	~80,236
This PR (5 runs, latest tip)	120,087 / 122,461 / 116,360 / 122,196 / 123,654
Delta	~+45% to +54% (typical ~+50% around 120–123k)

Other simple.js variants (same tip):

Config	RPS (3 runs)
chunkedEnc=1 (TE: chunked)	121k / 120k / 116k
len=1024 content-length	103k / 106k / 96k

Other microbenches

Workload	RPS
end-vs-write-end method=end asc/64 c=50 duration=5	~104–129k (run-to-run variance)
end-vs-write-end method=write asc/64 c=50 duration=5	~106k
Custom keep-alive res.end('ok') only (50 clients, 3s)	~47–49k

Review feedback (document flags, no incidental C++ reformat, no Buffer.concat) is included; the common ASCII path still shows the large win vs baseline.

Tests

Full test/parallel/test-http* suite: 751/751 passed (pre-push on an earlier tip); targeted suite re-run after the latest speed-ups.

Test plan

• tools/test.py test/parallel/test-http* --mode=release (751 passed)
• Targeted cases: write-head, non-utf8 content-disposition, chunked 204/304, CONNECT, client default headers, outgoing-end-types, hex/utf16le first-chunk encodings, standalone ServerResponse
• benchmark/http/simple.js before/after (and after review + further speed-ups)
• CI on this PR

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/net

[mcollina]

Amazing results!

[mcollina]

lgtm

This needs a CITGM run and some validation it doesn't break express, fastify, koa, etc before shipping.

[github-actions]

The notable-change PRs with changes that should be highlighted in changelogs. label has been added by @mcollina.

Please suggest a text for the release notes if you'd like to include a more detailed summary, then proceed to update the PR description with the text or a link to the notable change suggested text comment. Otherwise, the commit will be placed in the Other Notable Changes section.

[mcollina]

Also a benchmark CI run would be interesting.

[anonrig]

Benchmark CI: https://ci.nodejs.org/view/Node.js%20benchmark/job/benchmark-node-micro-benchmarks/1883/

[anonrig]

CITGM: https://ci.nodejs.org/view/Node.js-citgm/job/citgm-smoker/3741/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74699/

[codecov]

Codecov Report

❌ Patch coverage is 83.34901% with 177 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.19%. Comparing base (8fec65d) to head (1dd52f6).

Files with missing lines	Patch %	Lines
lib/_http_outgoing.js	85.45%	86 Missing and 20 partials ⚠️
src/node_http_parser.cc	78.74%	30 Missing and 41 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64393      +/-   ##
==========================================
- Coverage   90.25%   90.19%   -0.06%     
==========================================
  Files         741      741              
  Lines      241207   242197     +990     
  Branches    45430    45719     +289     
==========================================
+ Hits       217698   218456     +758     
- Misses      15084    15239     +155     
- Partials     8425     8502      +77     

Files with missing lines	Coverage Δ
src/node_http_parser.cc	83.05% <78.74%> (-1.82%)	⬇️
lib/_http_outgoing.js	90.30% <85.45%> (-7.32%)	⬇️

... and 34 files with indirect coverage changes

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