fix(Makefile): use native curl and tar to bypass leanSpec key download bug

[Cyberking99]

Description

This PR fixes an issue where make test-spec mysteriously fails with an Aborted! error when attempting to download pre-generated XMSS test keys from GitHub.

The failure stems from an upstream bug in leanSpec's Python-based test key downloader, where an unflushed temporary file buffer causes an EOFError during extraction (which the click CLI then swallows and outputs as Aborted!).

To bypass this, this PR updates the leanSpec/fixtures target in the Makefile to:

1. Dynamically resolve the download URL via Python.
2. Use curl to securely stream the archive safely to /tmp.
3. Use the native tar utility to reliably extract the files.
4. Append -n auto to the uv run fill command to execute fixture generation in parallel, massively speeding up the process.

Associated Issue

Closes #334

Test Plan

1. Cleared any existing test keys inside the leanSpec workspace.
2. Ran make test-spec.
3. Verified that curl successfully downloaded the prod_scheme.tar.gz and tar extracted it correctly into the test_keys directory.
4. Verified that uv run fill detected the keys, bypassed its internal download function, and successfully generated the fixtures in parallel.

Checklist

• I have read the CONTRIBUTING.md (if applicable)
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes