Fix #1050: useField returns Form initialValues on first render

Issue: In v7.0.0, useField returns undefined on first render even when Form
initialValues are set. This breaks apps trying to upgrade from v6.

Root cause: useField's useState initializer falls back to creating initial
state when getFieldState() returns undefined, but only checks field-level
initialValue, not the Form's initialValues.

Solution: In the fallback case, check form.getState().initialValues[name]
before using field-level initialValue. Form-level initialValues take
precedence.

Tests:
- useField returns Form initialValues on first render
- useField uses field initialValue when Form initialValues missing that field
- All existing tests pass (14/14)

This is a clean solution that avoids the side effects of synchronous
registration during render (which React forbids).

Author: Erik Rasmussen

src/useField.test.js

@@ -508,4 +508,46 @@ describe("useField", () => {
     expect(calls).toContain("test"); // At least one call with 'test'
     expect(calls[calls.length - 1]).toBe(null); // Last call is null
   });
+
+  it("should return Form initialValues on first render (fix #1050)", () => {
+    const renderSpy = jest.fn();
+    const MyField = () => {
+      const { input } = useField("username");
+      renderSpy(input.value);
+      return <input {...input} data-testid="username" />;
+    };
+    const { getByTestId } = render(
+      <Form onSubmit={onSubmitMock} initialValues={{ username: "erikras" }}>
+        {() => (
+          <form>
+            <MyField />
+          </form>
+        )}
+      </Form>,
+    );
+    // Critical: on the FIRST render, value should be "erikras" not undefined
+    expect(renderSpy.mock.calls[0][0]).toBe("erikras");
+    expect(getByTestId("username").value).toBe("erikras");
+  });
+
+  it("should use field initialValue when Form initialValues doesn't have that field (fix #1050)", () => {
+    const renderSpy = jest.fn();
+    const MyField = () => {
+      const { input } = useField("username", { initialValue: "fieldLevel" });
+      renderSpy(input.value);
+      return <input {...input} data-testid="username" />;
+    };
+    const { getByTestId } = render(
+      <Form onSubmit={onSubmitMock} initialValues={{ other: "value" }}>
+        {() => (
+          <form>
+            <MyField />
+          </form>
+        )}
+      </Form>,
+    );
+    // Field-level initialValue should be used as fallback
+    expect(renderSpy.mock.calls[0][0]).toBe("fieldLevel");
+    expect(getByTestId("username").value).toBe("fieldLevel");
+  });
 });

src/useField.ts

@@ -113,9 +113,15 @@ function useField<
       return existingFieldState;
     }
 
+    // FIX #1050: Check Form initialValues before falling back to field initialValue
     // If no existing state, create a proper initial state
-    let initialStateValue = initialValue;
-    if (component === "select" && multiple && initialValue === undefined) {
+    const formState = form.getState();
+    const formInitialValue = formState.initialValues?.[name as keyof FormValues];
+    
+    // Use Form initialValues if available, otherwise use field initialValue
+    let initialStateValue = formInitialValue !== undefined ? formInitialValue : initialValue;
+    
+    if (component === "select" && multiple && initialStateValue === undefined) {
       initialStateValue = [];
     }