test(sync): add cross-element matching tests, enable <em> KEPUB conversion test

cfi_converter_test.go:
- TestFindTextInNode_SingleTextNode: baseline single-node match
- TestFindTextInNode_CrossEmElement: 'Vokalia and Consonantia' across
  two <em> elements — verifies match returns the 'Vokalia' text node
- TestFindTextInNode_CrossStrongElement: text crossing <strong> boundary
- TestFindTextInNode_DoesNotCrossParagraphs: verifies block boundary
  enforcement — text split across <p> elements is NOT matched
- TestFindTextInNode_NestedFormatting: <em><strong> nesting
- TestFindBlockParent: verifies findBlockParent walks up through inline
  elements to find block-level <p>
- TestCollectInlineText: verifies text segments are collected in order
  with correct content

kepub_cfi_converter_test.go:
- TestKEPUBConvertWithEmElements: previously skipped, now expects exact
  precision and verifies round-trip conversion works for text spanning
  <em> element boundaries

internal/sync/kepub_cfi_converter_test.go

@@ -424,13 +424,14 @@ func TestKEPUBConvertWithEmElements(t *testing.T) {
 
 	epubNode, epubOffset := findTextInNode(epubBody, searchText)
 	if epubNode == nil {
-		t.Skip("text not found in EPUB (may span elements)")
+		t.Fatalf("findTextInNode should find text spanning <em> elements: %q", searchText)
 	}
 
 	standardCFI, err := buildCFI(0, epubNode, epubOffset)
 	if err != nil {
 		t.Fatalf("build CFI: %v", err)
 	}
+	t.Logf("Standard CFI (em): %s", standardCFI)
 
 	result, err := converter.ConvertStandardCFIToKEPUB(standardCFI, 0.2, searchText)
 	if err != nil {
@@ -441,6 +442,17 @@ func TestKEPUBConvertWithEmElements(t *testing.T) {
 	if result.CFI == "" {
 		t.Error("expected non-empty CFI")
 	}
+	if result.Precision != "exact" {
+		t.Errorf("expected exact precision for cross-element text, got %s", result.Precision)
+	}
+
+	backResult, err := converter.ConvertKEPUBCFIToStandard(result.CFI, 0.2, searchText)
+	if err != nil {
+		t.Fatalf("KEPUB→standard round-trip: %v", err)
+	}
+	if backResult.Precision != "exact" {
+		t.Errorf("expected exact precision on round-trip, got %s", backResult.Precision)
+	}
 }
 
 func TestKEPUBConvertInvalidCFI(t *testing.T) {