fix: keep a repeater frame until the radio queue takes it

The frame was popped before queueTx(), so whenever the radio queue was busy
it was lost.  Peek it instead, only remove it once it is queued (or TX is
suspended), and don't try while the queue has no room.  The repeater buffer
moves to TxBuffer.h/.cpp so it can be unit tested.

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

src/TxBuffer.h

@@ -0,0 +1,43 @@
+#ifndef TX_BUFFER_H
+#define TX_BUFFER_H
+#include <Arduino.h>
+
+// Repeater frames waiting to be handed to the radio task.  Kept free of the
+// radio and timers so it can be unit tested on the PC (pio test -e native).
+
+#define MAX_TX_BUFFER 5
+
+struct somfy_tx_t {
+  void clear() {
+    this->hwsync = 0;
+    this->bit_length = 0;
+    memset(this->payload, 0x00, sizeof(this->payload));
+  }
+  uint8_t hwsync = 0;
+  uint8_t bit_length = 0;
+  uint8_t payload[10] = {};
+};
+struct somfy_tx_queue_t {
+  somfy_tx_queue_t() { this->clear(); }
+  void clear() {
+    for (uint8_t i = 0; i < MAX_TX_BUFFER; i++) {
+      this->index[i] = 255;
+      this->items[i].clear();
+    }
+    this->length = 0;
+  }
+  unsigned long delay_time = 0;  // Set by the caller, not before this millis().
+  uint8_t length = 0;
+  uint8_t index[MAX_TX_BUFFER] = {255};
+  somfy_tx_t items[MAX_TX_BUFFER];
+  // The oldest frame without removing it, so it can stay queued until the radio
+  // takes it.
+  bool peek(somfy_tx_t *tx) const;
+  bool pop(somfy_tx_t *tx);
+  // When full the oldest frame is thrown away.
+  void push(uint8_t hwsync, byte *payload, uint8_t bit_length);
+  private:
+    // Position in index of the oldest frame, -1 when empty.
+    int8_t oldest() const;
+};
+#endif