fix: symmetric umlaut/ß folding in lexical path + eval digraph case

Smoke-testing invented German dictations exposed that `ae`/`oe`/`ue`
spellings collapsed lexical recall (e.g. "Hyperlipidaemie" → garbage,
"Hyperlipidämie" → correct). normalize() now folds ä→ae, ö→oe, ü→ue,
ß→ss; it is applied only in lexical.rs (both corpus index and query),
so the path is symmetric under either spelling. The embedding path
(raw text for bge-m3) is intentionally untouched.

eval.rs inject_errors case 2 changed from umlaut-deletion (ä→a) to the
realistic ASCII-digraph variant (ä→ae) so the eval harness exercises —
and guards against regression of — exactly this failure class.

TDD: failing tests first; normalize_tests 5/5, full suite 44/0.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/normalize_tests.rs

@@ -11,7 +11,29 @@ fn expands_common_medical_abbreviations() {
     assert_eq!(normalize("Z.n. OP"), "zustand nach operation");
 }
 
+// The system must match regardless of whether the dictation uses real
+// umlauts (ä, ö, ü, ß) or their ASCII digraphs (ae, oe, ue, ss). Both the
+// indexed corpus text and the query pass through normalize(), so folding
+// both forms to one canonical (ASCII-digraph) representation makes the
+// lexical path symmetric — "Hyperlipidämie" and "Hyperlipidaemie" match.
 #[test]
-fn keeps_german_umlauts() {
-    assert_eq!(normalize("Ödem"), "ödem");
+fn folds_umlauts_and_eszett_to_ascii_digraphs() {
+    assert_eq!(normalize("Ödem"), "oedem");
+    assert_eq!(normalize("Fußödem"), "fussoedem");
+    assert_eq!(normalize("über"), "ueber");
+}
+
+#[test]
+fn umlaut_and_digraph_forms_normalize_identically() {
+    assert_eq!(normalize("Hyperlipidämie"), normalize("Hyperlipidaemie"));
+    assert_eq!(normalize("Ödem"), normalize("Oedem"));
+    assert_eq!(normalize("Fußödem"), normalize("Fussoedem"));
+    assert_eq!(normalize("über"), normalize("ueber"));
+}
+
+#[test]
+fn folding_is_idempotent_on_already_ascii_text() {
+    // ASCII-only medical text must be unchanged by the fold.
+    assert_eq!(normalize("hyperlipidaemie"), "hyperlipidaemie");
+    assert_eq!(normalize("Diabetes Mellitus"), "diabetes mellitus");
 }