✅ Cover fallback autocompletes

When a fallback argument is present, the second overload supplies the
contextual type, so the handler map popup offers optional keys (`a? b? c?`)
and handled keys stay optional (`b? c?`). The exhaustive one-argument popup
is unchanged. Covered for both factories.

src/primitive.test.ts

@@ -344,12 +344,14 @@ interface LabelsProbe {
     readonly name: string;
     readonly factory: "getMatcher" | "getMatcherW";
     readonly body: string;
+    readonly tail?: string;
 }
 
 const labelsFor = ({
     name,
     factory,
     body,
+    tail = "",
 }: LabelsProbe): Promise<readonly string[]> => {
     const session = new LspSession(REPO_ROOT);
     const target: CompletionTarget = {
@@ -358,7 +360,7 @@ const labelsFor = ({
             `import { ${factory} } from "./index.ts";`,
             `const m = ${factory}<${UNIVERSE}>()({`,
             body,
-            "});",
+            `}${tail});`,
             "",
         ].join("\n"),
     };
@@ -402,6 +404,42 @@ test("autocomplete: handled keys drop out of the popup", () => {
     });
 });
 
+test("autocomplete: a fallback makes the remaining keys optional", () => {
+    // Arrange
+    const name = "getMatcher_with_fallback";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getMatcher",
+        body: "    /*COMPLETE*/",
+        tail: ", () => 0",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["a?", "b?", "c?"]);
+    });
+});
+
+test("autocomplete: with a fallback, handled keys stay optional", () => {
+    // Arrange
+    const name = "getMatcher_with_fallback_after_key";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getMatcher",
+        body: "    a: () => 1,\n    /*COMPLETE*/",
+        tail: ", () => 0",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["b?", "c?"]);
+    });
+});
+
 test("autocomplete: `getMatcherW` offers the same popup as `getMatcher`", () => {
     // Arrange
     const name = "getMatcherW_fresh";
@@ -418,3 +456,21 @@ test("autocomplete: `getMatcherW` offers the same popup as `getMatcher`", () =>
         assert.deepEqual([...result], ["a", "b", "c"]);
     });
 });
+
+test("autocomplete: `getMatcherW` also offers optional keys with a fallback", () => {
+    // Arrange
+    const name = "getMatcherW_with_fallback";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getMatcherW",
+        body: "    /*COMPLETE*/",
+        tail: ", () => 0",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["a?", "b?", "c?"]);
+    });
+});