SmartSpellChecker: frequency-based boundary repair for valid word pairs

Previously, boundary repair was skipped when both words were valid
dictionary words (e.g., "Pound sand", "wit hit", "done euro").
Now uses word-frequency scoring (product of bigram frequencies) to
decide if the repair produces a more common word pair.

Threshold: repair accepted when new pair is >5x more frequent, or
when repair produces a known abbreviation.

New fixes: Pound sand→Pounds and (2000x), wit hit→with it (100000x),
done euro→one euro (7x).

43 tests passing.

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

klausur-service/backend/tests/test_smart_spell.py

@@ -166,8 +166,8 @@ class TestBoundaryRepair:
         result = sc.correct_text("be good ats th.", "en")
         assert "at sth." in result.corrected, f"Expected 'at sth.' in '{result.corrected}'"
 
-    def test_no_repair_if_both_known(self, sc):
-        """Don't repair if both words are already valid."""
+    def test_no_repair_common_pair(self, sc):
+        """Don't repair if both words form a common pair."""
         result = sc.correct_text("at the", "en")
         assert result.corrected == "at the"
         assert not result.changed
@@ -184,6 +184,21 @@ class TestBoundaryRepair:
         assert repair[0] == "at"
         assert repair[1] == "sth."
 
+    def test_pound_sand_to_pounds_and(self, sc):
+        """'Pound sand' → 'Pounds and' — both valid but repair is much more frequent."""
+        result = sc.correct_text("Pound sand euros", "en")
+        assert "Pounds and" in result.corrected, f"Expected 'Pounds and' in '{result.corrected}'"
+
+    def test_wit_hit_to_with_it(self, sc):
+        """'wit hit' → 'with it' — frequency-based repair."""
+        result = sc.correct_text("be careful wit hit", "en")
+        assert "with it" in result.corrected, f"Expected 'with it' in '{result.corrected}'"
+
+    def test_done_euro_to_one_euro(self, sc):
+        """'done euro' → 'one euro' in context."""
+        result = sc.correct_text("done euro", "en")
+        assert "one euro" in result.corrected, f"Expected 'one euro' in '{result.corrected}'"
+
 
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