feat(ocap-kernel): name the failing vat when a subcluster vat fails to launch

[grypez]

Closes #564
Closes #572

Explanation

When a vat in a subcluster fails to launch — most notably when its buildRootObject throws, surfacing from VatHandle.make as Failed to initialize vat <vatId>: ... — the error previously propagated through #launchVatsForSubcluster verbatim, giving no indication of which ClusterConfig vat failed (the internal kernel id alone is meaningless to someone reading a cluster config).

VatManager.launchVat now wraps runVat and, on failure, re-throws attributing the failure to the specific vat by both its kernel id and its ClusterConfig name, preserving the original error as the cause:

Failed to launch vat v3 (bob)

This is the only seam where both the minted vatId and the vatName are in scope. SubclusterManager.launchSubcluster already rolls back the subcluster and rethrows on the failure, so the named error surfaces to the caller.

This addresses the "throw an error describing which vats failed to buildRootObject" ask from #564. It also confirms (via new regression tests) that the original cryptic symptom — SES_UNHANDLED_REJECTION: no record matching key 'queue.run.3' (#572) — no longer occurs: the bootstrap subroutines reject cleanly and roll back.

Changes

• VatManager.launchVat: attribute launch failures to <vatId> (<vatName>), with the original error as cause; omit the parenthetical when launched without a name.
• Tests: VatManager.test.ts covers the attribution, cause preservation, and no-name cases. SubclusterManager.test.ts adds #572 regression tests for clean bootstrap-failure propagation and rollback.

References

• Closes feat(ocap-kernel): Add error handling for #launchVatsForSubcluster #564 (primary ask), refs feat(ocap-kernel): Make #launchVatsForSubcluster throw when its subroutines cause an uncaught promise rejection #572

Checklist

• I've clearly explained what changes were made and why.
• I've added tests to cover my changes.
• I've applied the appropriate labels (breaking / non-breaking).
• I've communicated my changes in the changelog.

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