fix: DKG goroutine and instance lifecycle (v3.1.0)

[olegshmuelov]

Summary

Fixes v3.1.0 QA 2.6 "goroutine cleanup — memory leaks still persist." Threads a lifecycle context through LocalOwner + instWrapper, replaces kyber's TimePhaser with a cancel-aware version, and adds a background reaper so expired instances release heap pressure under sparse traffic.

What's fixed

• PostReshare <-o.done deadlock on the success path — nothing ever sent to o.done, so every successful reshare leaked its WaitEnd goroutine. sync.Once-guarded close now used across PostDKG, PostReshare, broadcastError.
• Orphaned bchan senders on timeout — broadcast closure exits via instanceCtx.Done().
• WaitEnd watchers outliving the instance — runWaitEnd races WaitEnd() against ctx.Done() with a 5s grace for late completions.
• kyber TimePhaser residue (~30s) — replaced with cancellablePhaser in pkgs/wire/phaser.go; kyber exits within one phase signal.
• Instance lifecycle: Instance.Close() plumbed into ProcessMessage timeout, cleanInstances, validateInstances; ProcessMessages honors the lifecycle ctx.
• Heap retention: Switch.StartReaper sweeps expired entries every 30s.

Scope

In: goroutine lifecycle, kyber cancellation, instance eviction, heap-retention reaper.

Deferred (future releases): error-code unification (1.4 / 1.5 / 2.3), Pong version (4.1), body-limit tightening (2.1), atomic MaxInstances cap, CLI, Docker/CI, streaming SSZ.

Known limitations

• Post-cancel broadcast can rarely deliver a legitimate result to an in-flight caller (no leak; real work).
• Under extreme concurrent admissions, MaxInstances=1024 may briefly overshoot by the number of in-flight admission goroutines.

Test plan

• go test -race -timeout 300s ./pkgs/... — all pass
• golangci-lint run ./pkgs/... — 0 issues
• New: pkgs/wire/phaser_test.go, pkgs/dkg/lifecycle_test.go, pkgs/operator/lifecycle_test.go; goleak verifies kyber residue clears on cancel via TestDKGCancelReleasesKyberGoroutines
• QA 2.8 EIP-1271 integration tests re-run
• Full v3.1.0 QA re-pass under sustained + timeout-storm traffic