http: avoid dictionary-mode objects in responses

[mcollina]

setHeader() stored headers in a { __proto__: null } literal and OutgoingMessage let EventEmitter create the same literal for its listener table. V8 creates null-prototype literals in dictionary mode, so every response paid for a hash table allocation on the first setHeader()/on() call, for runtime-miss stores (the profile shows StoreIC::Store, Factory::NewStoreHandler and TieringManager::NotifyICChanged in steady state) and for dictionary lookups on every header access, including the for...in in _storeHeader().

This PR uses a fast-mode object with an empty null-prototype chain for the headers (same keys, same enumeration, no inherited properties), and presets the listener table with the common events, as Readable/Writable already do.

Hello-world server (res.setHeader('Content-Type', ...) + res.end('Hello World')), server pinned to one core, wrk -t2 -c50 on two other physical cores, 4 interleaved rounds:

	req/s	user µs/req	sys µs/req	alloc B/req
main	40.9k	15.0	9.4	4374
this PR	50.0k	11.3	8.7	3905

The _events preset accounts for ~2% of that; the header container for the rest. With 8 request headers and a handler that reads two of them: 36.2k → 43.2k req/s. Responses are byte-identical.

How much the dictionary-mode container costs depends on the response pattern (same harness, 2 rounds each):

handler	main	this PR
setHeader('Content-Type') + end(body)	41.7–42.4k	51.3–51.8k
setHeader('Content-Type') + setHeader('Content-Length') + end(body)	47.1–49.6k	50.0–51.7k
writeHead(200, { ... }) + end(body) (no setHeader(), benchmark/fixtures "normal")	46.1–50.0k	50.2–50.9k

The existing benchmark/http fixtures either set Content-Length through setHeader() or pass every header to writeHead(), so they only show the smaller gains. The second commit adds a setHeaderImplicit case to http/set-header.js (res.setHeader('Content-Type', ...) + res.end(body), framing left to end()). benchmark/compare.js --runs 10 --filter set-header http:

                                                                        confidence improvement accuracy (*)   (**)   (***)
http/set-header.js duration=5 res='normal' benchmarker='wrk'                            2.53 %       ±4.03% ±5.61%  ±7.86%
http/set-header.js duration=5 res='setHeader' benchmarker='wrk'                 **      6.13 %       ±4.03% ±5.55%  ±7.61%
http/set-header.js duration=5 res='setHeaderImplicit' benchmarker='wrk'        ***     16.46 %       ±5.10% ±7.06%  ±9.78%
http/set-header.js duration=5 res='setHeaderWH' benchmarker='wrk'                       4.78 %       ±5.76% ±7.89% ±10.75%

simple.js, headers.js and incoming_headers.js are unchanged within noise.

—-

Ai generated, humanly reviewed.

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/net
• @nodejs/performance

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.38%. Comparing base (8bf7793) to head (3aacac4).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66420      +/-   ##
==========================================
- Coverage   90.39%   90.38%   -0.01%     
==========================================
  Files         792      792              
  Lines      275580   275704     +124     
  Branches    52840    52857      +17     
==========================================
+ Hits       249104   249193      +89     
- Misses      16897    16937      +40     
+ Partials     9579     9574       -5     

Files with missing lines	Coverage Δ
lib/_http_outgoing.js	97.98% <100.00%> (+0.02%)	⬆️

... and 35 files with indirect coverage changes

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