fix(proxy): switch selector on every set-aside kind, not quota only

[maxmad64bis]

⚠️ base-red inherited: #15306

Summary

A pool member set aside for a transport failure or a slow response kept serving its selector group. Only quota refusals steered the local core selector away. Every set-aside now steers the selector to a live member.

Related Issues

• Related to [defer] feat(proxy): steer local core selector off set-aside members #14752

Validation

• Change type: provider
• Focused tests and category gates from the golden path
• npm run lint — ESLint on the touched files is clean; the full run is red on the base (🔴 Release branch not green: release/v3.8.52 #15306)
• Reconciled with the current active release base; focused checks rerun afterward
• Production-code changes include a new or updated automated test in this PR

Tests Added Or Updated

• tests/unit/proxySubscription.selectorTransition.test.ts (new, 5 tests: collapsed quota double trigger, gap-window refusal, one switch per transport/slow kind with kind-observable member memory, idempotent registration, end-to-end transport flip)
• tests/unit/proxySubscription.selectorTrigger.test.ts, selectorMemberMemory, selectorWarning, selectorFunctional (existing, green)

Coverage Notes

• src/lib/proxySubscription/selectorTrigger.ts — covered by proxySubscription.selectorTransition.test.ts and proxySubscription.selectorTrigger.test.ts
• src/lib/proxySubscription/proxyTransitionSubscriber.ts — covered by proxySubscription.selectorTransition.test.ts
• src/instrumentation-node.ts — wiring only, covered indirectly by proxySubscription.selectorTransition.test.ts
• src/sse/handlers/proxyOutcomeMemory.ts — unchanged (synchronous quota path kept as-is)

Reviewer Notes

• Inherited from the base (🔴 Release branch not green: release/v3.8.52 #15306): mutation-test-coverage and docs-all fail the same way on the base and on this branch, files outside this PR are unchanged. Either order with feat(proxy-subscription): generate the local proxy-core configuration (opt-in) #15141: both touch selectorTrigger.ts (this change passes the refusal kind explicitly, feat(proxy-subscription): generate the local proxy-core configuration (opt-in) #15141 adds the snapshot fallback — same behavior either order).
• Follow-up: feat(admin): expose selector member set-aside state in pool visibility #15240 exposes the per-member set-aside state in the pool visibility route (same route, new accessor).

[diegosouzapw]

Thanks @maxmad64bis — merged into release/v3.8.52; it ships in the next release.