Avoid blocking auto-standby holds on persistence

[sjmiller609]

summary

• move auto-standby runtime metadata writes outside the controller state mutex
• serialize writes per instance and discard superseded updates so persisted runtime follows controller mutation order
• preserve operation-specific persistence error context
• add regression coverage for blocked persistence, cross-instance isolation, superseded generations, and instance deletion

why

Runtime metadata persistence can wait on instance storage while holding the controller-wide mutex. That delays unrelated auto-standby holds and adds latency before browser traffic can be forwarded.

tests

• go test -race -count=10 ./lib/autostandby
• go vet ./lib/autostandby
• go test ./... (partial: packages unrelated to this change fail because embedded binaries and mkfs.erofs are unavailable; lib/autostandby passes)

---

Note

Medium Risk
Concurrency and ordering changes around idle/standby runtime metadata affect recovery across restarts and hold latency; behavior is localized to autostandby with targeted race tests but storage timing can still diverge briefly from in-memory state.

Overview
Auto-standby runtime persistence no longer holds the controller-wide mutex while SetRuntime runs, so slow instance storage cannot block HoldStandby or work on other instances.

State updates still happen under the lock via prepareRuntimePersistenceLocked, which stamps a generation and a per-instance serialization lock; persistRuntime runs afterward, skips stale generations and deleted instances, and uses propagate vs best-effort error handling with operation labels in logs.

Call sites across seed/resync, connection events, standby confirmation, standby execution, and active reconcile were refactored to unlock before persisting; connection handling can batch multiple persist jobs after one locked pass.

Regression tests cover cross-instance non-blocking, latest-generation wins, deleted-instance skip, and hold completing while persistence is in flight.

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