The orphan thin check runs only after a failed snapshot backup (R-1012 follow-up, unreleased)

v0.159.0/v0.159.1 ran the check before every snapshot-mode whole-guest backup, which takes a
metadata snapshot of the live pool. Now explainFailure (vzdump start or task failure) is its only
caller; the finding is still appended to the failed backup's error. Tests: failure = 1 call,
success = 0 calls; red-proved against the v0.159.1 runner.go (2 calls / calls=1).

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

internal/capability/manifest.go

@@ -91,7 +91,7 @@ var manifest = []Capability{
 	{"disk-mkfs-xfs", "guarded format (xfs)", "/usr/local/sbin/felhom-mkfs-guarded", []string{"/dev/sda", "xfs"}, false, ""},
 	{"disk-smart", "disk SMART health read", "/usr/sbin/smartctl", []string{"-a", "-j", "/dev/sda"}, false, ""},
 	{"disk-lvs", "thin-pool usage read", "/usr/sbin/lvs", []string{"--reportformat", "json", "--units", "b", "-o", "lv_name,data_percent,metadata_percent", "--", "pve/data"}, false, ""},
-	{"thin-orphan-check", "orphan thin device check around a whole-guest backup (read-only, R-1012)", "/usr/local/sbin/felhom-thin-check", []string{"pve", "data"}, false, ""},
+	{"thin-orphan-check", "orphan thin device check after a failed whole-guest backup (read-only, R-1012)", "/usr/local/sbin/felhom-thin-check", []string{"pve", "data"}, false, ""},
 
 	// ---- Storage mount units (watchdog re-mount) ----
 	{"mount-unit-install", "fs-UUID mount unit install (root content check, R-861)", "/usr/local/sbin/felhom-priv-apply", []string{"unit", "mnt-felhom\\x2dx.mount"}, false, ""},