[Tony133]

Proposal:

Follow-up to fastify/fast-json-stringify#906, as discussed there and in fastify/fastify#6507.

Context:

fast-json-stringify@7.3.0 added objectMechanism: 'native': the serializer projects the payload into schema-shaped objects and delegates to JSON.stringify, so on Node.js 25+ V8 stays on its fast path while the response schema is still applied. Output is identical to the default mechanism; the option is a no-op on Node.js < 25.

Change:

The compiler already forwards serializerOpts as-is, and the types import Options from fast-json-stringify, so nothing in the code needs to change. This PR makes the option usable and visible:

• fast-json-stringify dependency bumped to ^7.3.0, so objectMechanism is accepted and typed;
• README: short section with the serializerOpts: { objectMechanism: 'native' } example;
• tests: an invalid objectMechanism throws through the compiler (proves the option reaches fast-json-stringify on every runtime), 'native' and 'default' give the same output on an input carrying an undeclared property and a Date, and a Fastify integration test with serializerOpts: { objectMechanism: 'native' };
• type tests: 'native' and 'default' accepted, an arbitrary string rejected.

Why now:

This is the first step towards proposing 'native' as the default in Fastify v6; it lets users opt in on v5 today.

Note:

• This raises the dependency floor to fast-json-stringify@^7.3.0 and documents a new option, so it should go out as a minor release (semver-minor).
• min-release-age=7 rejects fast-json-stringify@7.3.0 until Nov 12. The second commit temporarily excludes it from the check (min-release-age-exclude, npm >= 11.17), with a dated comment in .npmrc; I'll remove those two lines after the 12th, or drop the commit now if you'd rather wait.

[mcollina]

lgtm