So even importing `fcntl` causes problems on windows. This PR replaces
fcntl with portalocker. Tests all pass, so I think we are good.
ref:
https://arcade-ai.slack.com/archives/C08K1SJ072S/p1767897850450239?thread_ts=1766186586.406019&cid=C08K1SJ072S
<img width="934" height="501" alt="Screenshot 2026-01-08 at 2 57 46 PM"
src="https://github.com/user-attachments/assets/1375b6b2-116c-44bd-bbe1-2157dd243d29"
/>
Closes ENGTOP-8
<!-- CURSOR_SUMMARY -->
---
> [!NOTE]
> **Cross-platform file locking**
>
> - Replace `fcntl` with `portalocker` in
`arcade_core/usage/identity.py` (shared/exclusive locks); switch to
atomic `os.replace()`
> - Add `portalocker` dependency and bump `arcade-core` to `4.2.1`
>
> **Installation/CI**
>
> - New GitHub Actions workflow `test-install.yml` runs install/CLI
checks on macOS, Windows, and Linux for Python 3.10/3.12
> - Add `tests/install/test_install.py` and README to verify install,
`arcade` CLI availability, and `portalocker` locking behavior
>
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
3fe98fbcbf177f51fdb0b7fc51b20060f7fc85ad. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->
c034046735