Scroll speeds below 1 dot per second

scroll_speed is now a float (0.05-30 dots per second, e.g. 0.25 = one dot
every 4 s). Page format 5; version 4 pages are converted, version 3 goes
through version 4.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/test_esp32/test_main.cpp

@@ -1401,6 +1401,34 @@ static bool anyDot(const uint8_t *b, int x0, int x1)
 	return false;
 }
 
+void test_slow_scrolling_below_one_dot_per_second()
+{
+	FakeResources res;
+	DynamicPage page = scrollPage();
+	page.elements[0].scroll_speed = 0.25f; // a dot every 4 s
+	TEST_ASSERT_NULL(validatePage(page));
+	uint8_t a[BITMAP_BYTES], b[BITMAP_BYTES];
+	TEST_ASSERT_EQUAL(4000, renderPage(page, at(12, 0), 0, res, a));
+	TEST_ASSERT_EQUAL(1, renderPage(page, at(12, 0), 3999, res, b));
+	TEST_ASSERT_EQUAL(0, memcmp(a, b, BITMAP_BYTES)); // not moved yet
+	TEST_ASSERT_EQUAL(4000, renderPage(page, at(12, 0), 4000, res, b));
+	TEST_ASSERT_TRUE(memcmp(a, b, BITMAP_BYTES) != 0); // one dot on
+	TEST_ASSERT_EQUAL(1500, renderPage(page, at(12, 0), 6500, res, b));
+
+	page.elements[0].scroll_speed = 0.3f; // 3333.3 ms per dot: no drift over a long time
+	TEST_ASSERT_EQUAL(3334, renderPage(page, at(12, 0), 0, res, a));
+	uint32_t t = 3000000000UL; // ~35 days of millis()
+	uint32_t wait = renderPage(page, at(12, 0), t, res, a);
+	TEST_ASSERT_TRUE(wait >= 1 && wait <= 3334);
+
+	page.elements[0].scroll_speed = 0.01f; // too slow
+	TEST_ASSERT_NOT_NULL(validatePage(page));
+	page.elements[0].scroll_speed = NAN;
+	TEST_ASSERT_NOT_NULL(validatePage(page));
+	page.elements[0].scroll_speed = 31;
+	TEST_ASSERT_NOT_NULL(validatePage(page));
+}
+
 void test_scrolling_text_moves_in_its_window()
 {
 	FakeResources res;
@@ -2297,6 +2325,7 @@ int main()
 	RUN_TEST(test_bar_render);
 	RUN_TEST(test_bar_validation);
 	RUN_TEST(test_plex_user_placeholder);
+	RUN_TEST(test_slow_scrolling_below_one_dot_per_second);
 
 	return UNITY_END();
 }