Refactor DMX UART mapping and port usage

Introduces a new `dmx::port::Info` model (`dmx_port.h`) to centralize per-port UART/GPIO/usage metadata, then rewires DMX/RDM IRQ, DMA, direction, and send paths to derive port indices from UART at compile time with uniqueness checks. This removes legacy `DmxPortToUart` mapping, adds TX/RX-only behavior guards, and updates direction/data handling to respect per-port usage.

The change also modernizes UART helpers under `gd32::` (`UartBegin`, flag/interrupt helpers, `gd32::Uart` enum), fixes `has_continuous_output_` naming, and introduces reusable E1.20 message-length constants used in RDM parsing.

Author: vanvught

lib-dmx/include/gd32/dmx.h

@@ -120,7 +120,7 @@ class Dmx {
     uint32_t transmit_length_[dmx::config::max::kPorts];
     uint16_t transmit_slots_{dmx::kChannelsMax};
     dmx::Direction port_direction_[dmx::config::max::kPorts];
-    bool has_continuos_output_{false};
+    bool has_continuous_output_{false};
 
     inline static Dmx* s_this;
 };

lib-dmx/include/gd32/dmx_port.h

@@ -0,0 +1,23 @@
+/**
+ * @file dmx_port.h
+ *
+ */
+
+#ifndef GD32_DMX_PORT_H_
+#define GD32_DMX_PORT_H_
+
+#include <cstdint>
+#include "gd32_uart.h"
+
+namespace dmx::port {
+enum class Usage { kTxRx = 0, kTxOnly = 1, kRxOnly = 2 };
+
+struct Info {
+    gd32::Uart uart;
+    uint32_t port;
+    uint32_t pin;
+    Usage usage;
+};
+} // namespace dmx::port
+
+#endif // GD32_DMX_PORT_H_