Audit DBHandle N-API references reachable from cross-environment shutdown

[kriszyp]

Parent: #741

Problem

PR #780 removes the transaction-owned jsDatabaseRef, making TransactionHandle::close() N-API-free. DBHandle still owns environment-bound references, including transaction-log entries in logRefs, and DBHandle::close() deletes those references unconditionally. That close path is reachable from shared-descriptor and registry shutdown, where the calling thread or teardown phase may not match the environment that created each reference.

PR #745 added a teardown guard around the DBHandle::close() loop, but the PR was closed and #780 does not carry an equivalent audit or fix.

Required invariant

Every environment-owned N-API reference reachable from process-global or cross-environment native state must be released only through a lifecycle path that is valid for its owning environment. Cross-environment and late-teardown cleanup must not call N-API through a stale napi_env.

Acceptance criteria

• Enumerate DBHandle and adjacent shutdown paths that create, read, or delete environment-owned N-API references.
• Establish ownership and allowed-thread/allowed-phase rules for each reference.
• Prefer environment-scoped release or structural removal of the native reference over scattered thread-identity checks.
• Verify DBRegistry::Shutdown, shared DBDescriptor::finishClose, user-called db.close(), GC finalization, and worker environment teardown.
• Add a worker test that exercises transaction-log references across shared-database environment churn and last-environment shutdown.

Relationship

• Umbrella: Worker-env teardown destroys transactions on the shared DBDescriptor, corrupting the heap under concurrent commits #741
• Transaction-owned reference removal: fix: close pending transactions with their owning DBHandle; napi-free TransactionHandle::close (#741) #780
• Previous partial implementation/reference: fix: N-API use-after-free during worker-env teardown, plus two VT/transaction lifecycle races (#741) #745

— KrAIs (Codex), on behalf of @kriszyp