Clarify advertising TX power handling and work around Bumble unsigned-int bug.

src/auracast/multicast.py

@@ -520,7 +520,17 @@ async def init_broadcast(
                 advertising_sid=i,
                 primary_advertising_phy=hci.Phy.LE_1M, # 2m phy config throws error - because for primary advertising channels, 1mbit is only supported
                 secondary_advertising_phy=hci.Phy.LE_1M, # this is the secondary advertising beeing send on non advertising channels (extendend advertising)
-                advertising_tx_power=global_config.advertising_tx_power, # dBm; clamped to nearest SDC-supported step (max via CONFIG_BT_CTLR_TX_PWR_*)
+                # Pass NO_PREFERENCE (0x7F) here for two reasons:
+                # 1. The Nordic SoftDevice Controller ignores this field for
+                #    advertising sets and always returns the compile-time
+                #    CONFIG_BT_CTLR_TX_PWR_* value. The real TX power is
+                #    applied via the Zephyr VS Write_Tx_Power_Level command
+                #    issued right after create_advertising_set() returns.
+                # 2. Bumble's HCI metadata declares this field as 1-byte
+                #    *unsigned* (a bumble bug — the BT spec defines it as
+                #    signed int8), so negative values would raise
+                #    "bytes must be in range(0, 256)" at serialization.
+                advertising_tx_power=hci.HCI_LE_Set_Extended_Advertising_Parameters_Command.TX_POWER_NO_PREFERENCE,
                 #secondary_advertising_max_skip=10,
             ),
             advertising_data=(
@@ -537,9 +547,11 @@ async def init_broadcast(
             auto_start=True,
         )
         bigs[f'big{i}']['advertising_set'] = advertising_set
-        logging.info(
-            'Advertising TX power (LE_Set_Ext_Adv_Params): requested=%+d dBm, controller selected=%+d dBm (handle=%d)',
-            global_config.advertising_tx_power,
+        # NOTE: selected_tx_power below reflects the SDC's compile-time max
+        # (LE_Set_Ext_Adv_Params was sent with NO_PREFERENCE). The actual
+        # transmit power is set by the VS Write_Tx_Power_Level call below.
+        logging.debug(
+            'LE_Set_Ext_Adv_Params reports controller fallback TX power: %+d dBm (handle=%d)',
             getattr(advertising_set, 'selected_tx_power', 0),
             i,
         )