Add CHANGELOG documenting the formula syntax fix

Co-authored-by: Kenearos <86194771+Kenearos@users.noreply.github.com>

CHANGELOG.md

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+# Changelog
+
+## 2025-11-14 - Fix Excel Formula Syntax Error
+
+### Issue
+Fixed a syntax error in the Checks sheet that would cause Excel formula errors.
+
+### Details
+The nested WENN (IF) formula in the Checks sheet Status column was missing a semicolon between the empty string result and the second WENN function.
+
+**File:** `src/build_template.py`, line 264
+
+**Before:**
+```excel
+=WENN(A2="";""WENN(ABS(B2-1)<=0,0001;"OK";"FEHLER"))
+```
+
+**After:**
+```excel
+=WENN(A2="";"";WENN(ABS(B2-1)<=0,0001;"OK";"FEHLER"))
+```
+
+### Impact
+This fix ensures that the Checks sheet Status column works correctly to validate that daily shift totals sum to 1.0, displaying "OK" or "FEHLER" as appropriate.
+
+### Testing
+- Template rebuilt successfully
+- November 2025 file generated without errors
+- All formulas verified syntactically correct
+- No security issues found (CodeQL scan: 0 alerts)
+
+### How to Use
+To generate a corrected November 2025 file:
+
+```bash
+# 1. Rebuild the template with the fix
+python src/build_template.py
+
+# 2. Generate November 2025
+python src/fill_plan_dates.py 2025 11
+
+# 3. Open output/Dienstplan_2025_11_NRW.xlsx in Excel
+```
+
+The file will now work correctly with all formulas calculating as expected according to the Variante 2 (streng) rules specified in SPECIFICATION.md.