fix: Relax SearchResult validation to support DBSF fusion scores > 1.0

Fix false-positive validation error where DBSF (Distribution-Based Score
Fusion) correctly produces scores > 1.0 but SearchResult validation
incorrectly rejected them.

**Root Cause**: SearchResult.__post_init__() enforced scores in [0.0, 1.0]
range, but DBSF sums normalized scores from multiple retrieval systems
(dense semantic + sparse BM25), resulting in scores like 1.55 when both
systems strongly agree a document is relevant.

**Changes**:
- Relaxed validation to allow any score ≥ 0.0 (algorithms.py:147-157)
- Updated SearchResult and SemanticSearchResult documentation to explain
  score ranges for RRF ([0.0, 1.0]) vs DBSF (unbounded)
- Added comprehensive test coverage for both fusion methods
- Added DBSF fusion option to vector visualization UI
- Updated viz routes and vizApp() to support fusion parameter selection

**Testing**: All 157 unit tests pass, type checking passes, ruff passes

Fixes error: "Configuration error: Score must be between 0.0 and 1.0, got 1.1528953"

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

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

tests/unit/search/__init__.py

@@ -0,0 +1 @@
+"""Unit tests for search algorithms."""

tests/unit/search/test_search_result.py

@@ -0,0 +1,135 @@
+"""Unit tests for SearchResult validation."""
+
+import pytest
+
+from nextcloud_mcp_server.search.algorithms import SearchResult
+
+
+@pytest.mark.unit
+def test_search_result_rrf_score_in_range():
+    """Test SearchResult accepts RRF scores in [0.0, 1.0] range."""
+    result = SearchResult(
+        id=1,
+        doc_type="note",
+        title="Test Note",
+        excerpt="Test excerpt",
+        score=0.85,
+    )
+
+    assert result.score == 0.85
+
+
+@pytest.mark.unit
+def test_search_result_rrf_score_at_lower_bound():
+    """Test SearchResult accepts RRF score at lower bound (0.0)."""
+    result = SearchResult(
+        id=1,
+        doc_type="note",
+        title="Test Note",
+        excerpt="Test excerpt",
+        score=0.0,
+    )
+
+    assert result.score == 0.0
+
+
+@pytest.mark.unit
+def test_search_result_rrf_score_at_upper_bound():
+    """Test SearchResult accepts RRF score at upper bound (1.0)."""
+    result = SearchResult(
+        id=1,
+        doc_type="note",
+        title="Test Note",
+        excerpt="Test excerpt",
+        score=1.0,
+    )
+
+    assert result.score == 1.0
+
+
+@pytest.mark.unit
+def test_search_result_dbsf_score_above_one():
+    """Test SearchResult accepts DBSF scores > 1.0.
+
+    DBSF (Distribution-Based Score Fusion) sums normalized scores from multiple
+    systems (dense semantic + sparse BM25), so scores can exceed 1.0 when both
+    systems strongly agree a document is relevant.
+    """
+    # Typical DBSF score when both systems agree
+    result = SearchResult(
+        id=1,
+        doc_type="note",
+        title="Highly Relevant Note",
+        excerpt="Contains keywords and is semantically similar",
+        score=1.55,
+    )
+
+    assert result.score == 1.55
+
+
+@pytest.mark.unit
+def test_search_result_dbsf_score_edge_case():
+    """Test SearchResult accepts DBSF maximum theoretical score (2.0).
+
+    Maximum DBSF score with 2 systems: 1.0 (dense) + 1.0 (sparse) = 2.0
+    """
+    result = SearchResult(
+        id=1,
+        doc_type="note",
+        title="Perfect Match",
+        excerpt="Perfect semantic and keyword match",
+        score=2.0,
+    )
+
+    assert result.score == 2.0
+
+
+@pytest.mark.unit
+def test_search_result_negative_score_raises_error():
+    """Test SearchResult rejects negative scores."""
+    with pytest.raises(ValueError) as exc_info:
+        SearchResult(
+            id=1,
+            doc_type="note",
+            title="Test Note",
+            excerpt="Test excerpt",
+            score=-0.1,
+        )
+
+    assert "Score must be non-negative" in str(exc_info.value)
+    assert "got -0.1" in str(exc_info.value)
+
+
+@pytest.mark.unit
+def test_search_result_with_metadata():
+    """Test SearchResult with optional metadata field."""
+    result = SearchResult(
+        id=1,
+        doc_type="note",
+        title="Test Note",
+        excerpt="Test excerpt",
+        score=1.25,
+        metadata={"fusion_method": "dbsf", "dense_score": 0.8, "sparse_score": 0.45},
+    )
+
+    assert result.score == 1.25
+    assert result.metadata["fusion_method"] == "dbsf"
+    assert result.metadata["dense_score"] == 0.8
+    assert result.metadata["sparse_score"] == 0.45
+
+
+@pytest.mark.unit
+def test_search_result_with_chunk_offsets():
+    """Test SearchResult with chunk offset information."""
+    result = SearchResult(
+        id=1,
+        doc_type="note",
+        title="Test Note",
+        excerpt="matching chunk text",
+        score=0.9,
+        chunk_start_offset=100,
+        chunk_end_offset=500,
+    )
+
+    assert result.chunk_start_offset == 100
+    assert result.chunk_end_offset == 500