[metalwarrior665]

reqwest's http3 feature makes every client that uses use_preconfigured_tls build an HTTP/3 connector, even when HTTP/3 is never used. That means a quinn endpoint per client: a UDP socket, a driver task, and a zeroed receive buffer of at least 2 MB.

In a CPU profile of a Rust Instagram scraper that builds one impit client per proxy session, this endpoint setup took ~6% of all CPU, for a protocol it never uses.

HTTP/3 support is now behind a new http3 feature of the impit crate:

• With the feature off, clients skip the QUIC setup, and hickory-* and tokio aren't needed.
• The crate builds without --cfg reqwest_unstable.
• impit-node and impit-python enable the feature, so their http3 option behaves as before.
• For Rust users, with_http3() now needs features = ["http3"]. It's removed from the README and doc examples.

CI now also runs cargo test -p impit --features http3, and the docs are built with the feature on.

The proxy fix that used to be part of this PR is now #598. Whichever of the two lands second needs #[cfg(feature = "http3")] on #598's test, which calls with_http3().

🤖 Generated with Claude Code

https://claude.ai/code/session_0148RYR4BTGyCYHENSRZjC3S

[Copilot]

🟢 Approval recommended

The feature gates, binding configuration, documentation, and CI changes are consistent, with no blocking issues identified.

No open issues 🎉

What changed in this PR

Makes HTTP/3 opt-in for the Rust crate, avoiding unnecessary QUIC setup for default clients while preserving HTTP/3 support in the Node.js and Python bindings.

Changes:

• Adds an http3 Cargo feature and gates HTTP/3-specific code and dependencies.
• Updates examples and documentation to reflect the new opt-in requirement.
• Adds feature-enabled CI tests and includes HTTP/3 APIs in generated documentation.

File	Description
README.md	Documents HTTP/3 opt-in and simplifies the example.
impit/​src/​lib.rs	Gates the HTTP/3 module and updates crate documentation.
impit/​src/​impit.rs	Gates HTTP/3 functionality and tests default-disabled behavior.
impit/​README.md	Updates feature requirements and the example.
impit/​examples/​basic.rs	Removes HTTP/3 from the default example.
impit/​Cargo.toml	Defines the feature and makes supporting dependencies optional.
impit-python/​Cargo.toml	Enables HTTP/3 for Python bindings.
impit-node/​Cargo.toml	Enables HTTP/3 for Node.js bindings.
.github/​workflows/​format.yaml	Adds tests with HTTP/3 enabled.
.github/​workflows/​docs.yaml	Builds documentation with HTTP/3 enabled.

_{Review effort: Balanced}