Port gRPC global subchannel pool lifecycle

Tracks copied Bun corpus position 3980 from #66: js/third_party/grpc-js/test-global-subchannel-pool.test.ts.

Acceptance criteria:

• support EchoService waitForReady with exactly-once callback settlement
• keep client closed state independent when multiple clients share a target/global pool
• allow the remaining client to issue unary calls after a peer closes
• make repeated close idempotent and retain target-aware readiness errors
• lower the mixed grpc value/type imports, pass all three upstream checks, and pin the file permanently

Parent: #66

Completed in 0b95785d313dc0dfe69ef6861d810d4497112b57 and pushed to main.

• Added per-Echo-client readiness and closed state with exactly-once waitForReady completion and target-aware unavailable errors.
• Client close is idempotent and no longer affects peer clients sharing the same target/global transport configuration.
• The surviving client continues to perform real in-memory unary dispatch after its peer closes.
• Added narrow mixed-import/type lowering and pinned the upstream file permanently.
• Focused result: 3 passed, 0 todo, 0 failed, 0 unsupported; diagnostics, SQL, and the permanent third-party suite pass.