test(search): pin native Unicode substring matching

The shared literal owner already applies Unicode simple folding. Verify the reported production-helper cases across insensitive literal, sensitive literal and regex selection.

Fixes #6634

src/domain/analysisSearch.test.ts

@@ -1,6 +1,9 @@
 import { expect, it } from "vitest";
 
-import { literalSubstringPredicate } from "./analysisSearch.js";
+import {
+  analysisSearchPredicate,
+  literalSubstringPredicate,
+} from "./analysisSearch.js";
 
 it("uses Unicode simple case folding for literal substrings", () => {
   const matchesSigma = literalSubstringPredicate("Σ", false);
@@ -11,3 +14,40 @@ it("uses Unicode simple case folding for literal substrings", () => {
   expect(literalSubstringPredicate("A.B", false)("a.b")).toBe(true);
   expect(literalSubstringPredicate("A.B", false)("axb")).toBe(false);
 });
+
+it.each([
+  ["Σ", "ΟΣ"],
+  ["σ", "ΟΣ"],
+  ["ς", "ΟΣ"],
+  ["Σ", "ΣA"],
+  ["foo", "FOO"],
+  ["ΟΣ", "ΟΣ"],
+])(
+  "retains native literal match %s in %s when case is ignored",
+  (pattern, value) => {
+    expect(
+      analysisSearchPredicate({
+        pattern,
+        mode: "literal",
+        case_sensitive: false,
+        document: undefined,
+      })(value),
+    ).toBe(true);
+    expect(
+      analysisSearchPredicate({
+        pattern,
+        mode: "literal",
+        case_sensitive: true,
+        document: undefined,
+      })(value),
+    ).toBe(value.includes(pattern));
+    expect(
+      analysisSearchPredicate({
+        pattern,
+        mode: "regex",
+        case_sensitive: false,
+        document: undefined,
+      })(value),
+    ).toBe(true);
+  },
+);