fill watch: one warning per filesystem (R-1371) — committed, not released

On a box whose guest has one data volume the two built-in fill targets, "/"
(the controller's overlay on the Docker data volume) and /mnt/sys_drive, are one
filesystem. The target list was de-duplicated by path only, so one fill would
send two warnings under two names.

fillTargets now keeps one target per filesystem; the first stays (app area,
system area, drives). The identity is system.FilesystemKey, not st_dev alone:
inside the container "/" is an overlay with its own device number, so an
overlay path takes the device of a Docker-kept file on the data-root
(/etc/hostname, /etc/hosts, /etc/resolv.conf). Measured read-only on demo-hp
9201/9202 and the N100 on 2026-11-10.

Tests: TestR1371_* (3), TestFilesystemKey_* (2). Red seen three times.
No version, no image, no floor: it waits for a release session.

Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

controller/internal/system/fskey_test.go

@@ -0,0 +1,76 @@
+package system
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+)
+
+// R-1371 — the rule, with the numbers measured on demo-hp's guest 9201 (fskey.go).
+func TestFilesystemKey_AnOverlayRootIsTheDockerDataRoot(t *testing.T) {
+	devs := map[string]uint64{"/": 139, "/mnt/sys_drive": 64519, "/etc/hostname": 64519, "/etc/hosts": 64519, "/opt/docker/stacks": 64518}
+	ovl := map[string]bool{"/": true}
+	dev := func(p string) (uint64, bool) { d, ok := devs[p]; return d, ok }
+	overlay := func(p string) (bool, bool) {
+		if _, ok := devs[p]; !ok {
+			return false, false
+		}
+		return ovl[p], true
+	}
+	key := func(p string) (uint64, bool) { return filesystemKeyWith(p, dev, overlay, dockerDataRootAnchors) }
+
+	root, ok1 := key("/")
+	sys, ok2 := key("/mnt/sys_drive")
+	if !ok1 || !ok2 || root != sys {
+		t.Fatalf("/ (%d,%v) and /mnt/sys_drive (%d,%v) are ONE filesystem on every box measured — the keys must be equal", root, ok1, sys, ok2)
+	}
+	if stacks, ok := key("/opt/docker/stacks"); !ok || stacks == root {
+		t.Fatalf("/opt/docker/stacks (%d,%v) is another filesystem — its key must differ from %d", stacks, ok, root)
+	}
+	if _, ok := key("/mnt/not-mounted"); ok {
+		t.Fatal("an unreadable path must have no key")
+	}
+	if _, ok := key(""); ok {
+		t.Fatal("an empty path must have no key")
+	}
+
+	// No anchor readable → NO key (never the overlay's own number, which would look like a filesystem of its own
+	// only by luck, and never a guess).
+	if k, ok := filesystemKeyWith("/", dev, overlay, []string{"/etc/absent"}); ok {
+		t.Fatalf("an overlay with no readable anchor got key %d — want no key", k)
+	}
+	// An anchor that is itself on the overlay (not bind-mounted) names nothing; the next one is used.
+	devs["/etc/plain"] = 139
+	ovl["/etc/plain"] = true
+	if k, ok := filesystemKeyWith("/", dev, overlay, []string{"/etc/plain", "/etc/hosts"}); !ok || k != 64519 {
+		t.Fatalf("key = %d,%v — want the first anchor that is NOT on the overlay (64519)", k, ok)
+	}
+}
+
+// The real system calls: two paths on one filesystem share a key, a path on another filesystem does not, and an
+// absent path has none. Runs on the machine's real filesystems (and, in a container, through the overlay branch).
+func TestFilesystemKey_RealSyscalls(t *testing.T) {
+	if runtime.GOOS != "linux" {
+		t.Skip("the identity is Linux-only")
+	}
+	dir := t.TempDir()
+	sub := filepath.Join(dir, "sub")
+	if err := os.Mkdir(sub, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	a, okA := FilesystemKey(dir)
+	b, okB := FilesystemKey(sub)
+	if !okA || !okB || a != b {
+		t.Fatalf("a folder (%d,%v) and its sub-folder (%d,%v) must share a key", a, okA, b, okB)
+	}
+	if p, ok := FilesystemKey("/proc"); !ok || p == a {
+		t.Fatalf("/proc (%d,%v) is another filesystem than %s (%d)", p, ok, dir, a)
+	}
+	if k, ok := FilesystemKey(filepath.Join(dir, "absent")); ok {
+		t.Fatalf("an absent path got key %d", k)
+	}
+	if ov, ok := isOverlayFS("/proc"); !ok || ov {
+		t.Fatalf("/proc read as overlay=%v ok=%v", ov, ok)
+	}
+}