R-25 (agent half): the format answer carries the NEW filesystem's UUID, bound to the durable id

After mkfs the agent re-resolves the bound durable id, requires it to name the
device it just formatted, reads the superblock back (blkid -p, requested fstype)
and returns fs_uuid in POST /disks/format and GET /disks/format/status. Anything
unverified returns "" — never a path-resolved guess. The controller half
(mount fs_uuid instead of re-resolving the UUID from the /dev path) is owed
in felhom-controller.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

internal/localapi/disks.go

@@ -768,6 +768,11 @@ type FormatResponse struct {
 	// signature — the customer authorizes the wipe of their own data drive.
 	NeedsConfirmation bool   `json:"needs_confirmation,omitempty"`
 	DurableID         string `json:"durable_id,omitempty"` // the durable id to confirm against (user-data)
+	// FSUUID (on Formatted) is the UUID of the filesystem the agent just made, verified against the
+	// bound durable id after mkfs (R-25). The caller mounts THIS — re-resolving a UUID from the /dev
+	// path later can name another disk if /dev re-enumerated. "" = not verified: the caller must not
+	// substitute a path-resolved guess silently.
+	FSUUID string `json:"fs_uuid,omitempty"`
 	// PendingOp is set on a SYSTEM/BACKUP data-bearing refusal — the exact op the operator must sign.
 	PendingOp *PendingOp `json:"pending_op,omitempty"`
 }
@@ -815,7 +820,7 @@ func (s *Server) handleDiskFormatStatus(w http.ResponseWriter, r *http.Request,
 	writeOK(w, map[string]any{
 		"vmid": vmid, "phase": job.Phase, "device": job.Device, "fstype": job.FSType,
 		"durable_id": job.DurableID, "error": job.Error, "started_at": job.StartedAt, "updated_at": job.UpdatedAt,
-		"job_id": job.JobID,
+		"job_id": job.JobID, "fs_uuid": job.FSUUID, // R-25: "" until done + verified
 	})
 }
 
@@ -878,7 +883,7 @@ func (s *Server) handleDiskFormat(w http.ResponseWriter, r *http.Request, vmid i
 				"format refused (device may have changed since inspection): "+rerr.Error())
 			return
 		}
-		done := s.startFormatDetached(device, blankDurable, req.FSType, true)
+		job, done := s.startFormatDetached(device, blankDurable, req.FSType, true)
 		if err := s.awaitFormat(r.Context(), done, vmid, device); err != nil {
 			if err == errFormatClientGone {
 				return // client gone; mkfs continues detached + the job record records the outcome
@@ -887,7 +892,7 @@ func (s *Server) handleDiskFormat(w http.ResponseWriter, r *http.Request, vmid i
 			writeErr(w, http.StatusBadGateway, "format failed: "+err.Error())
 			return
 		}
-		writeOK(w, FormatResponse{VMID: vmid, Device: device, Formatted: true, DataBearing: false, DurableID: blankDurable, Reason: "blank device formatted " + req.FSType})
+		writeOK(w, FormatResponse{VMID: vmid, Device: device, Formatted: true, DataBearing: false, DurableID: blankDurable, FSUUID: job.FSUUID, Reason: "blank device formatted " + req.FSType})
 		return
 	}
 
@@ -924,7 +929,7 @@ func (s *Server) handleDiskFormat(w http.ResponseWriter, r *http.Request, vmid i
 		// F20-BUG3: run the destructive mkfs DETACHED off s.baseCtx (bound durable id recorded for
 		// restart-recovery), so a request/client deadline can never SIGKILL it mid-write and corrupt the
 		// disk. We still wait to return the synchronous result (backward-compatible with the controller).
-		done := s.startFormatDetached(device, deviceDurable, req.FSType, false)
+		job, done := s.startFormatDetached(device, deviceDurable, req.FSType, false)
 		if err := s.awaitFormat(r.Context(), done, vmid, device); err != nil {
 			if err == errFormatClientGone {
 				return // client gone; the wipe continues detached + survives a restart via the job record
@@ -936,7 +941,7 @@ func (s *Server) handleDiskFormat(w http.ResponseWriter, r *http.Request, vmid i
 		s.logger.Warn("local-api: USER-DATA data-bearing format — CUSTOMER CONFIRMED (no operator signature)",
 			"vmid", vmid, "device", device, "durable_id", deviceDurable, "fstype", req.FSType, "why", probe.Reason())
 		writeOK(w, FormatResponse{VMID: vmid, Device: device, Formatted: true, DataBearing: true,
-			Role: string(role), DurableID: deviceDurable, Reason: "customer-confirmed wipe (" + probe.Reason() + ")"})
+			Role: string(role), DurableID: deviceDurable, FSUUID: job.FSUUID, Reason: "customer-confirmed wipe (" + probe.Reason() + ")"})
 		return
 	}