refactor(config): parallelize test suite, serialize only stateful autostart tests

[StudentWeis]

Motivation

scripts/precheck.sh currently runs the entire test suite with --test-threads=1, forcing all ~363 tests through a single thread. Only a handful of tests actually share global state (operating-system-level auto-start registration in src/config/autostart.rs); the vast majority are already isolated via per-test tempfile directories or are pure logic tests. Forcing serial execution hurts local feedback time and CI throughput unnecessarily.

Proposed Solution

• Add serial_test as a dev-dependency.
• Annotate only the genuinely stateful tests (those in src/config/autostart.rs that touch OS-level auto-launch registration) with #[serial].
• Drop --test-threads=1 from scripts/precheck.sh so the remaining tests run in parallel.

Acceptance Criteria

• serial_test is present in [dev-dependencies].
• Auto-start tests are annotated with #[serial].
• scripts/precheck.sh no longer passes --test-threads=1.
• scripts/precheck.sh still passes locally end-to-end.

Scope

config