perf(ci): round-robin shard sequencer + parallel test:init

[VaibhavAcharya]

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Summary

Closing — measured improvement (~11% faster on PR-style ubuntu-only runs across 3+3 green runs) isn't worth shipping.

What was tried:

• Custom vitest sequencer doing alphabetical round-robin sharding (vs hash-based default)
• run-p test:init:* to parallelize fixture installs
• Bonus: removed a real concurrency race in clone.test.ts (describe.concurrent + let execaMock reassigned in beforeEach)

The clone test concurrency bug surfaced during this exploration is real and worth a separate PR. It will be filed independently.

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📊 Benchmark results

Comparing with 07fa4d1

• Dependency count: 1,061 (no change)
• Package size: 357 MB (no change)
• Number of ts-expect-error directives: 355 (no change)