feat(bt/ble_log): implement UART redirection stream write

Introduce stream write API with deferred frame encapsulation for UART
redirection on port 0. Redirected data is now properly framed (header +
payload + checksum) instead of being sent as raw ASCII, preventing frame
parser sync oscillation on the receiver.

- Add stream_seal/stream_write/stream_flush in ble_log_lbm.c
- Add BLE_LOG_SRC_REDIR source and BLE_LOG_UART_REDIR_ENABLED gate
- Simplify redir_uart_tx_chars and timer callback to use stream API
- Flush pending stream data in ble_log_prph_deinit
- Make get_trans static (no external callers after refactor)
- Move UART wrap linker flags outside CONFIG_BT_ENABLED guard
- Default UART DMA peripheral when SOC_UHCI_SUPPORTED
- Default baud rate 921600 -> 3000000

components/bt/common/ble_log/include/ble_log.h

@@ -35,6 +35,9 @@ typedef enum {
     BLE_LOG_SRC_HCI,
     BLE_LOG_SRC_ENCODE,
 
+    /* UART redirection (PORT 0 only) */
+    BLE_LOG_SRC_REDIR,
+
     BLE_LOG_SRC_MAX,
 } ble_log_src_t;