feat: persist seed and prngAlgorithm for exact problem reproducibility

Ensures that shared and auto-saved worksheets generate the exact same
problems when loaded:

Seed & Algorithm Persistence:
- Added seed and prngAlgorithm to extractConfigFields()
- Both fields now included in auto-save, sharing, and settings persistence
- prngAlgorithm defaults to 'mulberry32' (current implementation)

Type Updates:
- Added prngAlgorithm: string to WorksheetConfig (required)
- Added prngAlgorithm?: string to WorksheetFormState (optional)
- Updated extractConfigFields return type to include both fields

Problem Reproducibility:
- Same seed + same algorithm = exact same problems
- Critical for sharing worksheets with specific problem sets
- Allows reload to show same problems (via auto-save)
- Future-proofs for potential algorithm changes

Documentation Updates:
- Updated useWorksheetAutoSave comments to reflect seed persistence
- Clarified that seed/algo are critical for reproducibility, not transient

Bug Fix:
- Fixed broken link on /create page (/create/worksheets/addition → /create/worksheets)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

apps/web/src/app/create/page.tsx

@@ -283,7 +283,7 @@ export default function CreateHubPage() {
             </Link>
 
             {/* Worksheet Creator */}
-            <Link href="/create/worksheets/addition">
+            <Link href="/create/worksheets">
               <div
                 data-element="worksheets-card"
                 className={css({

apps/web/src/app/create/worksheets/hooks/useWorksheetAutoSave.ts

@@ -14,7 +14,8 @@ interface UseWorksheetAutoSaveReturn {
  *
  * Features:
  * - Debounced auto-save (1000ms delay)
- * - Only persists settings, not transient state (date, seed, rows, total)
+ * - Persists settings including seed and prngAlgorithm for problem reproducibility
+ * - Excludes transient state (date, rows, total)
  * - Persists V4 fields: mode, digitRange, displayRules, difficultyProfile, manualPreset
  * - Silent error handling (auto-save is not critical)
  * - StrictMode-safe (handles double renders)
@@ -45,7 +46,7 @@ export function useWorksheetAutoSave(
       console.log('[useWorksheetAutoSave] Attempting to save settings...')
       setIsSaving(true)
       try {
-        // Extract persisted config fields (excludes date, seed, derived state)
+        // Extract persisted config fields (includes seed/prngAlgorithm, excludes date and derived state)
         const config = extractConfigFields(formState)
 
         const response = await fetch('/api/worksheets/settings', {

apps/web/src/app/create/worksheets/types.ts

@@ -24,7 +24,10 @@ export type WorksheetConfig = AdditionConfigV4 & {
 
   // Personalization
   date: string
+
+  // Problem reproducibility (critical for sharing)
   seed: number
+  prngAlgorithm: string
 
   // Layout
   page: {
@@ -61,7 +64,9 @@ export type WorksheetFormState = Partial<Omit<AdditionConfigV4Smart, 'version'>>
     rows?: number
     total?: number
     date?: string
+    // Problem reproducibility (critical for sharing)
     seed?: number
+    prngAlgorithm?: string
   }
 
 /**

apps/web/src/app/create/worksheets/utils/extractConfigFields.ts

@@ -3,7 +3,8 @@ import type { WorksheetFormState } from '../types'
 
 /**
  * Extract only the persisted config fields from formState
- * Excludes derived state (rows, total, date, seed)
+ * Excludes derived state (rows, total, date)
+ * INCLUDES seed and prngAlgorithm to ensure exact problem reproduction when shared
  *
  * This ensures consistent field extraction across:
  * - Auto-save (useWorksheetAutoSave)
@@ -15,7 +16,7 @@ import type { WorksheetFormState } from '../types'
  */
 export function extractConfigFields(
   formState: WorksheetFormState
-): Omit<AdditionConfigV4, 'version'> {
+): Omit<AdditionConfigV4, 'version'> & { seed?: number; prngAlgorithm?: string } {
   return {
     problemsPerPage: formState.problemsPerPage!,
     cols: formState.cols!,
@@ -45,5 +46,8 @@ export function extractConfigFields(
     currentStepId: formState.currentStepId,
     currentAdditionSkillId: formState.currentAdditionSkillId,
     currentSubtractionSkillId: formState.currentSubtractionSkillId,
+    // CRITICAL: Include seed and algorithm to ensure exact same problems when sharing
+    seed: formState.seed,
+    prngAlgorithm: formState.prngAlgorithm ?? 'mulberry32',
   }
 }

apps/web/src/app/create/worksheets/validation.ts

@@ -148,6 +148,7 @@ export function validateWorksheetConfig(formState: WorksheetFormState): Validati
 
     fontSize,
     seed,
+    prngAlgorithm: formState.prngAlgorithm ?? 'mulberry32',
   }
 
   // Build mode-specific config