fix: serialize shim shutdown and quiesce guest before poweroff

[aledbf]

Summary

• Serialize shutdown flow: introduce beginRPC / shutdownOnce to prevent new RPCs during shutdown and ensure in-flight calls drain before exit
• Quiesce guest filesystem: add PrepareShutdown RPC so the host can flush the guest writable layer before VM poweroff, ensuring snapshot/commit consistency
• Bump toolchain and deps: Go 1.26.2, QEMU 10.2.2, containerd v2.2.2, gRPC v1.78.0, OpenTelemetry v1.38.0, EROFS snapshotter v20260419.01
• Remove stale api/next.pb.txt descriptor snapshot

Test plan

• Unit tests added for PrepareShutdown guest service
• Unit tests added for beginRPC / shutdown serialization in task service
• task lint passes
• go test -race ./... passes
• Integration tests with KVM verify clean shutdown under concurrent RPCs

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