fix: annotate asm.js integer return type

[Justineo]

AI disclosure — Codex, GPT-5.6 Sol (Extra High)

This PR was researched, implemented, tested, and drafted by Codex. I provided the requirements, requested alignment with project conventions, and approved publication.

Codex was asked to:

• find the regression boundary and root cause
• preserve behavior with a minimal fix
• audit Automatic asm.js dictionary extraction and clang toolchain update #203 for similar asm.js issues
• cover full and minimal builds
• prevent V8 fallback from hiding failures

Codex reviewed the build history, asm.js rules, and V8 behavior; changed return b to return b | 0; and added an explicit asm.js validation test.

Validation covered published versions 2.0.0–2.3.1, Node 16–25, full and minimal bundles, and behavior comparison across 20 source files. The complete chomp test matrix is pending CI approval.

Summary

Since 2.2.0, invoking the es-module-lexer/js build under V8 emits:

Invalid asm.js: Invalid return type

The bundle remains valid JavaScript, so V8 falls back to normal JavaScript compilation and parsing still succeeds. This allows the existing functional tests to pass while the asm.js module itself fails validation.

The regression was introduced by #203. Its injected allocator helper returns an integer parameter without the result coercion required by asm.js:

function su(a, b) {
  a = a | 0;
  b = b | 0;
  v = a + b + 15 & -16;
  return b;
}

Returning b is the intended behavior; only its asm.js return type annotation is missing.

Changes

• Change the allocator return to return b | 0
• Add a V8-specific regression test for both full and minimal asm.js builds
• Run validation in a child process with asm.js diagnostics enabled, so JavaScript fallback cannot hide validation or linking failures

The additional coercion does not change runtime behavior because b has already been converted with b = b | 0.

Test plan

• Confirm the regression test fails for the unmodified 2.3.1 full and minimal asm.js bundles
• Confirm the test passes for the patched full and minimal bundles
• Run JavaScript syntax and diff checks
• Run the complete chomp test matrix in CI with the Emscripten and Fastcomp toolchains