feat: integrate FallbackSteps into editor and session runner (Task 18)

Wire FallbackSteps edit mode into StepEditor for procedure_step type
with add/remove/update handlers using crypto.randomUUID(). Add execute
mode rendering in ProceduralNavigationPage with fallbackDecisions state
tracking per parent step.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/procedural-editor/StepEditor.tsx

@@ -2,6 +2,7 @@ import { useState } from 'react'
 import { ChevronUp, ChevronDown, AlertTriangle, Clock, ExternalLink, CheckSquare, Terminal, Settings2 } from 'lucide-react'
 import type { ProceduralStep, StepContentType, IntakeFormField } from '@/types'
 import { cn } from '@/lib/utils'
+import { FallbackSteps } from '@/components/procedural/FallbackSteps'
 
 const CONTENT_TYPE_OPTIONS: { value: StepContentType; label: string; color: string }[] = [
   { value: 'action', label: 'Action', color: 'text-blue-400' },
@@ -278,6 +279,32 @@ export function StepEditor({ step, stepNumber, onUpdate, onCollapse, availableVa
             </div>}
           </div>
         )}
+
+        {/* Fallback Steps — procedure_step only */}
+        {step.type === 'procedure_step' && (
+          <FallbackSteps
+            fallbackSteps={step.fallback_steps ?? []}
+            mode="edit"
+            onAdd={() => {
+              const newFallback: ProceduralStep = {
+                id: crypto.randomUUID(),
+                type: 'procedure_step',
+                title: '',
+              }
+              onUpdate({ fallback_steps: [...(step.fallback_steps ?? []), newFallback] })
+            }}
+            onRemove={(index) => {
+              const updated = (step.fallback_steps ?? []).filter((_, i) => i !== index)
+              onUpdate({ fallback_steps: updated.length > 0 ? updated : undefined })
+            }}
+            onUpdate={(index, updates) => {
+              const updated = (step.fallback_steps ?? []).map((fb, i) =>
+                i === index ? { ...fb, ...updates } : fb
+              )
+              onUpdate({ fallback_steps: updated })
+            }}
+          />
+        )}
       </div>
     </div>
   )

frontend/src/pages/ProceduralNavigationPage.tsx

@@ -5,7 +5,8 @@ import { treesApi } from '@/api/trees'
 import { sessionsApi } from '@/api/sessions'
 import { stepsApi } from '@/api/steps'
 import type { Tree, Session, ProceduralStep, DecisionRecord, RuntimeStep, CustomProceduralStep, IntakeFormField } from '@/types'
-import type { CustomStep } from '@/types/session'
+import type { CustomStep, FallbackStepRecord } from '@/types/session'
+import { FallbackSteps } from '@/components/procedural/FallbackSteps'
 import type { Step } from '@/types/step'
 import { StepChecklist } from '@/components/procedural/StepChecklist'
 import { StepDetail } from '@/components/procedural/StepDetail'
@@ -84,6 +85,9 @@ export function ProceduralNavigationPage() {
   const [batchProgress, setBatchProgress] = useState<{ completed: number; total: number } | null>(null)
   const timerRef = useRef<ReturnType<typeof setInterval> | null>(null)
 
+  // Fallback step decisions
+  const [fallbackDecisions, setFallbackDecisions] = useState<FallbackStepRecord[]>([])
+
   // Custom step state
   const [runtimeSteps, setRuntimeSteps] = useState<RuntimeStep[]>([])
   const [sessionCustomSteps, setSessionCustomSteps] = useState<CustomStep[]>([])
@@ -434,6 +438,22 @@ export function ProceduralNavigationPage() {
     setShowCsatModal(false)
   }
 
+  const handleFallbackComplete = (
+    parentStepId: string,
+    fallbackStepId: string,
+    notes: string | null,
+    outcome: 'resolved' | 'not_resolved' | 'skipped'
+  ) => {
+    const record: FallbackStepRecord = {
+      parent_step_id: parentStepId,
+      fallback_step_id: fallbackStepId,
+      completed_at: new Date().toISOString(),
+      notes,
+      outcome,
+    }
+    setFallbackDecisions((prev) => [...prev, record])
+  }
+
   const handleStepCreated = (step: Step | CustomStepDraft, isFromLibrary: boolean) => {
     setPendingCustomStep(step)
     setPendingIsFromLibrary(isFromLibrary)
@@ -734,6 +754,22 @@ export function ProceduralNavigationPage() {
             />
           )}
 
+          {/* Fallback steps — shown when step has fallback alternatives */}
+          {currentStep && !('isCustom' in currentStep && currentStep.isCustom) && 'fallback_steps' in currentStep && (
+            <FallbackSteps
+              fallbackSteps={(currentStep as ProceduralStep).fallback_steps ?? []}
+              mode="execute"
+              completedIds={new Set(
+                fallbackDecisions
+                  .filter((d) => d.parent_step_id === currentStep.id && d.outcome === 'resolved')
+                  .map((d) => d.fallback_step_id)
+              )}
+              onComplete={(fallbackStepId, notes, outcome) =>
+                handleFallbackComplete(currentStep.id, fallbackStepId, notes, outcome)
+              }
+            />
+          )}
+
           {/* Add custom step — only on current active incomplete non-custom step */}
           {currentStep && !completedStepIds.has(currentStep.id) && !('isCustom' in currentStep && currentStep.isCustom) && (
             <div className="mt-4">