refactor: make cel_serial_read non-blocking

Drop timeout_ms; read now returns immediately with whatever
data is available (0 if none), so callers don't block the rest
of their loop waiting on serial I/O.

telemetry's poll loop now sleeps interval_ms itself between
empty reads via a small sleep_ms helper.

tests/test_serial.c

@@ -34,7 +34,7 @@ void test_read_returns_zero_stub(void) {
     TEST_ASSERT_NOT_NULL(port);
     uint8_t buf[16];
     /* Stub implementation returns 0 */
-    size_t n = cel_serial_read(port, buf, sizeof(buf), 100);
+    size_t n = cel_serial_read(port, buf, sizeof(buf));
     TEST_ASSERT_EQUAL_UINT(0, n);
     cel_serial_close(port);
 }