fix: honor ValidModifierClass subset; inherit usage type; harden ordering

- Fix 1: capture <ValidModifierClass> codes from <ModifiedBy all="false">
  and filter each modifier level to only valid codes before cartesian
  product; eliminates ~194 phantom codes (e.g. M07.01–03) while keeping
  valid ones (M07.00/04/07/09). Split Start/Empty XML event arms to
  correctly track children of <ModifiedBy>.
- Fix 2: synthesized para295/para301 inherit parent values; "V" is
  promoted to "P" (parent "V" = needs subdivision, terminal is codeable).
  Other types ("P", "O", "Z", "") pass through unchanged.
- Fix 3: debug_assert in ModifierClass handler guards against ClaML
  document-order assumption changing silently in debug/test builds.
- Fix 4: add expands_single_modifier_chain regression test verifying
  C88.00/C88.01 synthesis from a single all="true" ModifiedBy (corpus-
  verified: Alpha-ID I30531, I31044).

tests/claml_modifier_tests.rs

@@ -14,3 +14,39 @@ fn expands_diabetes_5th_digit_codes_as_billable() {
     assert!(a010.description.contains("Typhus abdominalis"));
     assert_eq!(map.get("E11").expect("E11 root").para295, "V");
 }
+
+/// Regression test: a category with a single `<ModifiedBy all="true">` expands
+/// correctly into its terminal 5th-digit leaves.
+///
+/// `C88.0` (Makroglobulinämie Waldenström) has exactly one `<ModifiedBy
+/// all="true" code="S02C88_5"/>` with two `<ModifierClass>` codes ("0" and
+/// "1"), yielding `C88.00` and `C88.01`.  Both codes appear in the
+/// Alpha-ID corpus (`data/icd10gm2026_alphaidse_edvtxt_20250926.txt`).
+///
+/// The parent `C88.0` carries `Para295 = "P"` (already billable at the 4th
+/// digit) and `Para301 = "V"`.  The synthesized terminals therefore inherit
+/// `para295 = "P"` (parent value, not "V") and `para301 = "P"` ("V" promoted).
+#[test]
+fn expands_single_modifier_chain() {
+    let map = claml::load("icd-claml/Klassifikationsdateien/icd10gm2026syst_claml_20250912.xml").unwrap();
+
+    // C88.00 – verified in corpus (e.g. Alpha-ID I30531 "Makroglobulinämie Waldenström").
+    let c8800 = map.get("C88.00").expect("C88.00 synthesized from single ModifiedBy");
+    assert_eq!(c8800.para295, "P",
+        "para295 inherited from parent C88.0 (which is already P, not V)");
+    // Parent label + modifier label joined by " - ".
+    assert!(c8800.description.contains("Waldenström"),
+        "description must contain parent label: {:?}", c8800.description);
+    assert!(c8800.description.contains("Remission"),
+        "description must contain modifier label: {:?}", c8800.description);
+
+    // C88.01 – verified in corpus (Alpha-ID I31044 "Makroglobulinämie in kompletter Remission").
+    let c8801 = map.get("C88.01").expect("C88.01 synthesized");
+    assert_eq!(c8801.para295, "P");
+    assert!(c8801.description.contains("kompletter Remission"),
+        "description must include modifier label: {:?}", c8801.description);
+
+    // The 4th-digit parent itself must still be present and unchanged.
+    let c880 = map.get("C88.0").expect("C88.0 explicit parent still present");
+    assert_eq!(c880.para295, "P", "explicit parent entry must not be overwritten");
+}