Port gRPC pick-first load balancer

Scope

Port Bun corpus position 3986, js/third_party/grpc-js/test-pick-first.test.ts (22 active tests).

Required contracts

• preserve shuffle multisets and randomize selection when configured
• create and observe subchannels across endpoint/address lists
• implement CONNECTING, READY, IDLE, and sticky TRANSIENT_FAILURE transitions
• retain a READY subchannel while a replacement address list connects
• hand off to READY replacements and react correctly when the old subchannel disconnects
• request resolver updates for exhausted/failing address generations
• reconnect the same address list from exitIdle
• expose picker, load-balancer helper, address, and fixture surfaces
• preserve causes and add operation/address/source context for invalid updates

Verification

• all 22 focused upstream tests pass without unsupported cases
• permanent diagnostics cover invalid updates and state handoff
• existing gRPC, SQL adapter, and third-party gates remain green

Implemented and pushed in 8cfef6f11.

Completed contracts:

• shuffle preserves the input multiset and varies address selection when enabled
• endpoint/address lists create observable subchannels and READY pickers
• CONNECTING, READY, IDLE, and sticky TRANSIENT_FAILURE transitions follow pick-first lifecycle rules
• an established READY child remains active while a replacement list connects
• READY replacements hand off immediately; old-child disconnects evaluate the pending generation
• exhausted generations request resolver updates
• exitIdle reconnects the last address list
• process.nextTick now drains ahead of setImmediate, fixing the underlying Node event-loop ordering contract
• invalid addresses/helper failures produce ERR_GRPC_PICK_FIRST with preserved causes and operation/address/source stack context

Verification:

• focused Bun corpus: 22 passed, 0 failed, 0 todo, 0 unsupported
• permanent gRPC diagnostics: 13 passed
• SQL adapter contracts: passed
• permanent third-party JWT/utility mini-suite: passed
• zig fmt --check and git diff --check: passed

Cumulative through position 3986: 328 files, 2,741 passed, 209 todo, 0 failed, 0 unsupported, 171 allowed-empty.