fix: use correct Unicode minus sign (−) for subtraction operator checks

**Bug:** Subtraction problems were not getting their operator-specific display
rules in mixed mode because we were checking for ASCII hyphen '-' instead of
Unicode minus sign '−' (U+2212).

**Result:** All subtraction problems were falling back to default displayRules
instead of using subtractionDisplayRules, causing zero scaffolding.

**Fix:** Changed operator checks from '-' to '−' to match SubtractionProblem type.

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

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

apps/web/src/app/create/worksheets/addition/typstGenerator.ts

@@ -78,7 +78,7 @@ function generatePageTypst(
             `[TYPST PROBLEM ${index}] Using additionDisplayRules for ${p.a} + ${p.b}`,
             rulesForProblem
           )
-        } else if (p.operator === '-' && masteryConfig.subtractionDisplayRules) {
+        } else if (p.operator === '−' && masteryConfig.subtractionDisplayRules) {
           rulesForProblem = masteryConfig.subtractionDisplayRules
           console.log(
             `[TYPST PROBLEM ${index}] Using subtractionDisplayRules for ${p.minuend} - ${p.subtrahend}`,
@@ -89,7 +89,7 @@ function generatePageTypst(
 
       const displayOptions = resolveDisplayForProblem(rulesForProblem, meta)
 
-      if (p.operator === '-') {
+      if (p.operator === '−') {
         console.log(`[TYPST PROBLEM ${index}] Subtraction resolved display:`, {
           problem: `${p.minuend} - ${p.subtrahend}`,
           meta,