Summary

Propagates already-aborted input signals immediately when composing AbortSignals.

Linked issue

N/A

Changes

• Detect already-aborted signals before registering abort listeners.
• Reuse the existing abort path so cancellation cleanup and reason handling stay consistent.
• Add a regression test for composing a pre-aborted signal.

Validation

• npm run test:vitest:unit -- tests/unit/composeSignals.test.js tests/unit/helpers/composeSignals.test.js
• npm run lint -- lib/helpers/composeSignals.js
• git diff --check

Checklist

• Tests added or updated (or N/A with reason)
• Docs/types updated if public API changed (index.d.ts and index.d.cts)
• No breaking changes (or called out explicitly above)

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