Port Bun concurrent scheduling matrix

Track the next focused Bun compatibility tranche from #66.

Scope:

• run the byte-identical upstream js/bun/test/concurrent.test.ts corpus under Home
• preserve explicit concurrent boundaries and nested describe ordering
• model default/concurrent/serial behavior with and without --concurrent
• implement --max-concurrency 3, default cap, and zero/unbounded execution patterns
• retain Bun-compatible child reporter output
• add permanent Home regression coverage and run the full home_test gate

This issue will be closed by the direct-main implementation commit.

Completed in 48094fd (feat(test): port concurrent scheduling matrix). The byte-identical upstream concurrent.test.ts now passes all six cases under Home: explicit concurrency boundaries, nested describe ordering, --concurrent default-test behavior, serial overrides, --max-concurrency 3, the default cap of 20, and zero/unbounded growth. Reporter output is sourced from upstream inline snapshots where authoritative, and permanent aggregate coverage was added. Validation passed: exact corpus 6/6, focused regression, prior concurrent-immediate 2/2 regression, all four upstream byte comparisons, formatter/diff checks, and full home_test (1091 tests, 1 expected skip, 0 failures).