add aarch64-unknown-linux-gnu (linux-arm64) JS binding target

[Bl3f]

Summary

• Add aarch64-unknown-linux-gnu to the napi targets in package.json so the @pydantic/monty-linux-arm64-gnu optional dependency is generated
• Add a CI build job using GitHub's native ubuntu-24.04-arm ARM runner
• Add the target to the Linux JS binding test matrix (existing runs-on expression and docker params already handle aarch64 correctly)

Motivation

Docker on Apple Silicon (and any linux/arm64 environment) fails to load @pydantic/monty because the native binding for aarch64-unknown-linux-gnu is never built or published (related to #153):

Error: Cannot find native binding.
[cause]: Error: Cannot find module '@pydantic/monty-linux-arm64-gnu'

Reproduction:

FROM node:24-slim
WORKDIR /app
RUN npm install @pydantic/monty@0.0.4
RUN node -e "require('@pydantic/monty')"

[codspeed-hq]

Merging this PR will not alter performance

✅ 13 untouched benchmarks

---

_{Comparing Bl3f:add-linux-arm64-target (4589735) with main (135dcb0)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!