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• serialize writes per instance and discard superseded updates so persisted runtime follows controller mutation order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• add regression coverage for blocked persistence, cross-instance isolation, superseded generations, and instance deletion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)