node-api: use fast properties in node_api_create_object_with_properties

[colinhacks]

Description

Objects created with node_api_create_object_with_properties() are in dictionary mode, because the call uses v8::Object::New(). Their properties are about 7x slower to read than an object literal's, and about 70x slower with a NULL prototype, where each object also gets a map of its own.

This change keeps a cache of v8::DictionaryTemplate per env, keyed by a hash of the property names. Core already creates objects from dictionary templates in cares_wrap.cc and other places.

• A list of names passed once is only recorded by its hash.
• When the same list comes back, a template is created for it. Later objects with those names are instances of the template, with fast properties and one shared map.
• The first 256 lists that come back keep their template for the life of the env. Later lists keep using v8::Object::New().
• Symbols, strings that are not one-byte, array-index names, duplicate names and lists of more than 127 names also keep using v8::Object::New(), because a template cannot hold them.
• A prototype other than Object.prototype costs one SetPrototype() per object, as in NewDictionaryInstanceNullProto().

Fixes #66441

Benchmark

Measured with benchmark/compare.js, 30 runs per binary, both built from adcd028 on an Apple M1 Max (macOS 26.6) that was also running other work. The benchmark uses 20 properties and a NULL prototype. Every row below has p < 0.001 (Welch's t-test).

operation	n	before (ops/s)	after (ops/s)	after / before
create	100	906,183	788,484	0.87
create	1,000	826,700	1,128,979	1.37
create	10,000	1,234,144	950,498	0.77
create	100,000	1,338,166	699,711	0.52
create	1,000,000	529,317	400,732	0.76
read	100	1,427,576	2,501,004	1.75
read	1,000	3,420,600	11,972,203	3.50
read	10,000	5,376,966	19,301,626	3.59
read	100,000	6,765,430	57,977,854	8.57
read	1,000,000	6,834,671	73,790,321	10.80

The rows with method='old' (napi_create_object and napi_set_property) do not use the changed code. They moved by -1% to +7%.

Creation with a NULL prototype is slower because of the SetPrototype() call. With Object.prototype it is unchanged. The reproduction from #66441 measures both, with 10 properties and 100,000 objects, in ns per property (median of 3 runs; an object literal reads at 0.78 ns):

prototype	read before	read after	create before	create after
Object.prototype	5.67	0.72	38.4	39.3
NULL	53.08	0.84	41.6	62.2

Validation

• python3 tools/test.py --mode=release js-native-api node-api benchmark/test-benchmark-napi
• out/Release/cctest --gtest_filter='*NodeApi*:*Napi*'
• make lint-cpp, git-clang-format, and ESLint on the changed JS files
• The new test fails on the unpatched build at the first fast-properties check. It passed 300 runs in a row on the patched build.

---

AI disclosure: I used a coding agent for the investigation, patch, benchmarks, and PR text. It checked the template and Object::New() behavior against the V8 source, built Node.js, and ran the tests above. I approved the PR and will handle review comments myself.

[nodejs-github-bot]

Review requested:

• @nodejs/node-api
• @nodejs/performance

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[codecov]

Codecov Report

❌ Patch coverage is 87.65432% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.43%. Comparing base (1e9fd95) to head (0087afc).
⚠️ Report is 583 commits behind head on main.

Files with missing lines	Patch %	Lines
src/js_native_api_v8.cc	87.34%	2 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66470      +/-   ##
==========================================
+ Coverage   90.17%   90.43%   +0.26%     
==========================================
  Files         769      791      +22     
  Lines      261448   275644   +14196     
  Branches    49674    52861    +3187     
==========================================
+ Hits       235759   249292   +13533     
- Misses      16736    16757      +21     
- Partials     8953     9595     +642     

Files with missing lines	Coverage Δ
src/js_native_api_v8.h	91.05% <100.00%> (+0.14%)	⬆️
src/js_native_api_v8.cc	77.02% <87.34%> (+0.38%)	⬆️

... and 319 files with indirect coverage changes

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