Model gRPC local subchannel pool ownership

Scope

Port Bun corpus position 3982, js/third_party/grpc-js/test-local-subchannel-pool.test.ts, with the actual grpc.use_local_subchannel_pool ownership contract instead of treating the option as inert.

Required contracts

• local-pool clients acquire distinct subchannel records even for the same target
• global-pool clients share a target record and maintain reference counts
• closing one client cannot invalidate another client's pooled transport
• close is idempotent and releases the record exactly once
• invalid pool lifecycle operations retain causes and actionable pool/target stack context

Verification

• focused upstream file passes without unsupported cases
• permanent diagnostics distinguish local and global ownership
• existing gRPC, SQL adapter, and third-party gates remain green

Implemented and pushed in 41dd0e57e.

Completed contracts:

• grpc.use_local_subchannel_pool: 1 allocates isolated per-client subchannel records
• global-pool clients share target-scoped records with explicit reference counts
• closing one global client preserves the shared record for remaining clients
• close is idempotent and final release retires the record
• impossible/double-release invariants produce ERR_GRPC_SUBCHANNEL_POOL with a preserved cause and SubchannelPool.release, target, and source-module stack context

Verification:

• focused Bun corpus: 1 passed, 0 failed, 0 todo, 0 unsupported
• permanent gRPC diagnostics: 9 passed
• SQL adapter contracts: passed
• permanent third-party JWT/utility mini-suite: passed
• zig fmt --check and git diff --check: passed

Cumulative through position 3982: 324 files, 2,662 passed, 209 todo, 0 failed, 0 unsupported, 171 allowed-empty.