revert: undo per-crate --test-threads=1 split for musl CI

[branchseer]

Summary

Reverts fa42ef9 ("ci: replace global RUST_TEST_THREADS=1 with per-crate --test-threads=1 (#280)").

The change replaced the simple RUST_TEST_THREADS=1 env var with cargo-nextest installation and split test commands. This actually regressed musl CI performance:

CI Time Comparison (musl job)

	Before (7a8ad8b)	After (fa42ef9)	Delta
Test (musl)	4m 31s	12m 32s	+8m 01s (+177%)

The overhead comes from installing cargo-nextest and running two separate cargo invocations instead of one.

This revert restores the global RUST_TEST_THREADS=1 approach, bringing the musl CI job back to ~4.5 minutes.

Test plan

• Verify musl CI job passes
• Verify musl CI job completes in ~4-5 minutes instead of ~12 minutes

https://claude.ai/code/session_01LGzZqKECkrqipZfmmU9L3U

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