feat: auto-track fields on context for zero-arg ValidateAll/ResetFields

Fields created via Context.Field are now tracked on the page context,
so ValidateAll() and ResetFields() with no arguments operate on all
fields by default. Explicit field args still work for selective use.

Also switches MinLen/MaxLen to utf8.RuneCountInString for correct
unicode handling and replaces fmt.Errorf with errors.New where
format strings are unnecessary.

field_test.go

@@ -96,27 +96,22 @@ func TestFieldReset(t *testing.T) {
 
 func TestValidateAll(t *testing.T) {
 	v := New()
-	var username, email *Field
 	v.Page("/", func(c *Context) {
-		username = c.Field("", Required(), MinLen(3))
-		email = c.Field("", Required(), Email())
+		c.Field("", Required(), MinLen(3))
+		c.Field("", Required(), Email())
 		c.View(func() h.H { return h.Div() })
+
+		// both empty → both fail
+		assert.False(t, c.ValidateAll())
 	})
 
-	// both empty → both fail
-	assert.False(t, false) // sanity
-	ok := username.Validate() && email.Validate()
-	assert.False(t, ok)
-
-	// simulate ValidateAll via context
 	v2 := New()
-	var u2, e2 *Field
 	v2.Page("/", func(c *Context) {
-		u2 = c.Field("joe", Required(), MinLen(3))
-		e2 = c.Field("joe@x.com", Required(), Email())
+		c.Field("joe", Required(), MinLen(3))
+		c.Field("joe@x.com", Required(), Email())
 		c.View(func() h.H { return h.Div() })
 
-		assert.True(t, c.ValidateAll(u2, e2))
+		assert.True(t, c.ValidateAll())
 	})
 }
 
@@ -127,14 +122,30 @@ func TestValidateAllPartialFailure(t *testing.T) {
 		bad := c.Field("", Required())
 		c.View(func() h.H { return h.Div() })
 
-		// ValidateAll must run ALL fields even if first passes
-		ok := c.ValidateAll(good, bad)
+		ok := c.ValidateAll()
 		assert.False(t, ok)
 		assert.False(t, good.HasError())
 		assert.True(t, bad.HasError())
 	})
 }
 
+func TestValidateAllSelectiveArgs(t *testing.T) {
+	v := New()
+	v.Page("/", func(c *Context) {
+		a := c.Field("", Required())
+		b := c.Field("ok", Required())
+		cField := c.Field("", Required())
+		c.View(func() h.H { return h.Div() })
+
+		// only validate a and b — cField should be untouched
+		ok := c.ValidateAll(a, b)
+		assert.False(t, ok)
+		assert.True(t, a.HasError())
+		assert.False(t, b.HasError())
+		assert.False(t, cField.HasError(), "unselected field should not be validated")
+	})
+}
+
 func TestResetFields(t *testing.T) {
 	v := New()
 	v.Page("/", func(c *Context) {
@@ -146,13 +157,30 @@ func TestResetFields(t *testing.T) {
 		b.SetValue("changed-b")
 		a.AddError("err")
 
-		c.ResetFields(a, b)
+		c.ResetFields()
 		assert.Equal(t, "a", a.String())
 		assert.Equal(t, "b", b.String())
 		assert.False(t, a.HasError())
 	})
 }
 
+func TestResetFieldsSelectiveArgs(t *testing.T) {
+	v := New()
+	v.Page("/", func(c *Context) {
+		a := c.Field("a")
+		b := c.Field("b")
+		c.View(func() h.H { return h.Div() })
+
+		a.SetValue("changed-a")
+		b.SetValue("changed-b")
+
+		// only reset a
+		c.ResetFields(a)
+		assert.Equal(t, "a", a.String())
+		assert.Equal(t, "changed-b", b.String(), "unselected field should not be reset")
+	})
+}
+
 func TestFieldValidateClearsPreviousErrors(t *testing.T) {
 	f := newTestField("", Required())
 	f.Validate()