Hold what mounts a missing disk, and say entries the mesh could not take out (hq ADR 0325 re-review)

A container was still created over an unmounted mount point, so its writes landed on
the filesystem below; a folder moved to another disk was kept on the old warrant; and
entries left by a drop or a move that could not run lived only in a log line. Every
container mounting an access whose disk is away now waits, the warrant holds only for
the filesystem it was carried out on (the wait names the new one, path-free), entries
left on a filesystem are recorded per mount and taken out when it is back, and every
leftover is said as unhealthy until it is. A hung stat is shared per path, and a
widened mask takes from other entries only the bits it adds.

internal/store/kept.go

@@ -59,6 +59,14 @@ type KeptFolder struct {
 	// declared. Where they differ a walk adds or removes exactly the difference.
 	Users []KeptUser `json:"users,omitempty"`
 	Want  []KeptUser `json:"want,omitempty"`
+	// IDs are the accesses that last named the folder: what a leftover is said under once none does.
+	IDs []string `json:"ids,omitempty"`
+	// TakenOn is the mount point the operator's warrant was carried out on (ADR 0325 §8): the controller's word
+	// that the folder was taken holds for that filesystem alone, and a folder moved to another asks again.
+	TakenOn string `json:"taken_on,omitempty"`
+	// Left is the entries the mesh added on a filesystem no longer at the folder (a disk mounted over it, or
+	// one gone): taken out by a drop walk once that filesystem holds the folder again, said until then.
+	Left []KeptLeft `json:"left,omitempty"`
 	// Act is the last walk under the folder: a take, an add or a drop.
 	Act *KeptAct `json:"act,omitempty"`
 	// Sweep is how far the sweep got (ADR 0325 §10).
@@ -69,6 +77,12 @@ type KeptFolder struct {
 	OutOfWatches bool `json:"out_of_watches,omitempty"`
 }
 
+// KeptLeft is the mesh's entries left on one filesystem: its mount point and whose entries they are.
+type KeptLeft struct {
+	Mount string     `json:"mount"`
+	Users []KeptUser `json:"users"`
+}
+
 // KeptAct is one walk under a kept folder, as the engine records it. Counts only: never a path.
 type KeptAct struct {
 	// Act is take, add or drop.