Optimize SpiderMonkey fetch dispatch

[Copilot]

SpiderMonkey benchmarks were near parity with Boa because each request re-entered JavaScript through generated script evaluation. This changes the hot path to invoke the registered fetch dispatcher directly.

• SpiderMonkey dispatch

  • Replace per-request evaluate_script("__helios_call_fetch(...)") with JS_CallFunctionName.
  • Pass the serialized request as a rooted JS value instead of interpolating it into source text.

• Warmup path

  • Run JIT warmup through the same direct-call path used by real requests.
  • Remove generated JavaScript loop construction from warmup.

let args = HandleValueArray::from(req_arg.handle().into_handle());

JS_CallFunctionName(
    cx,
    global,
    CALL_FETCH_NAME.as_ptr() as *const std::os::raw::c_char,
    &args,
    rval.handle_mut(),
)

[VrilLabs]

Summary:

Optimized SpiderMonkey fetch dispatch by avoiding per-request evaluate_script.
Benchmarked SpiderMonkey ahead of Boa on dynamic handler:

• Boa: 75,941.875 rps
• SpiderMonkey: 86,582.25 rps

Validated:

cargo build
cargo test -p helios
cargo check -p helios --features spidermonkey
cargo fmt --check

parallel_validation: code review completed; CodeQL timed out per tool limit.

[Copilot]

Pull request overview

This PR wires an optional native SpiderMonkey backend into HELIOS and changes the SpiderMonkey fetch path to call the registered dispatcher directly rather than re-evaluating generated JavaScript per request.

Changes:

• Adds mozjs as the optional dependency behind the spidermonkey feature.
• Implements a SpiderMonkey-backed engine path in xdr.rs, including direct JS_CallFunctionName dispatch and warmup.
• Switches the CLI to select SpiderMonkey when the feature is enabled; several other files are formatting-only changes.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
rs/helios/src/xdr.rs	Adds the SpiderMonkey backend implementation and tests.
rs/helios/src/main.rs	Selects Boa or SpiderMonkey as the active CLI engine based on feature flags.
rs/helios/Cargo.toml	Adds optional mozjs dependency to the spidermonkey feature.
rs/Cargo.lock	Records the new SpiderMonkey dependency graph.
rs/helios/src/boa_backend.rs	Formatting-only changes.
rs/helios/src/wizer_build.rs	Formatting-only changes.
rs/helios/src/wit_host.rs	Formatting-only changes.
rs/helios/src/webtransport.rs	Formatting-only changes.

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