v0.317.0: a second copy counts only on a proven different disk; the no-drive install speaks in words (felhom.eu architecture/16, build A)

Tier-2 and the one-copy notice ask the host agent (POST /disks/same-disk, agent 0.157.0): different +
proven -> made and counted; same disk -> not made (D1); unknown (VM, hidden USB serial) -> made, not
counted, the row says why (D2); a NAS never counts and no longer hides the notice (R-1075). An older
agent keeps the st_dev rule, logged once. D4: a data-folder or media app on a box with no drive is
refused in words with a link to the drives page (R-1068's raw-error half). MinAgent 0.157.0.

Claude-Session: https://claude.ai/code/session_012qRErfCoiTkvDK9N5XHbzb

controller/internal/agentapi/features.go

@@ -88,6 +88,11 @@ const FeatureRetainedRecoveryClass Feature = "retained_recovery_class"
 // Unknown it stays, and an old agent's 404 is answered with the update sentence.
 const FeatureHostPowerOff Feature = "host_poweroff"
 
+// FeatureSameDisk — POST /disks/same-disk (agent v0.157.0, felhom.eu architecture/16-storage-physical-identity.md
+// §4): the agent's physical-disk verdict for two guest paths. Below it the Tier-2 runner keeps the old st_dev
+// rule (backup.Manager.diskRelation), logged once.
+const FeatureSameDisk Feature = "same_disk"
+
 // SupportState is a probe verdict. The zero value is SupportUnknown (fail-open: unknown never
 // refuses — the existing agent-error paths speak honestly when the agent is down).
 type SupportState int
@@ -168,6 +173,9 @@ var featureProbes = map[Feature]func(ctx context.Context, p SupportProber) error
 	FeatureHostPowerOff: func(ctx context.Context, p SupportProber) error {
 		return errNoRecoveryProbe
 	},
+	FeatureSameDisk: func(ctx context.Context, p SupportProber) error {
+		return errNoRecoveryProbe // not probeable: decided by the version header alone
+	},
 }
 
 // errNoMemoryProbe classifies to SupportUnknown (not a *StatusError 404), so a prober that cannot be
@@ -204,6 +212,8 @@ var featureMinAgent = map[Feature]string{
 
 	// R-998 — POST /host/poweroff, the household's „A gép kikapcsolása".
 	FeatureHostPowerOff: "0.155.0",
+
+	FeatureSameDisk: "0.157.0",
 }
 
 // MinAgentFor returns the declared minimum agent version for a feature ("" when the feature has no