Add NUS two-characteristic support to NimBLEStream for terminal app compatibility

[Copilot]

The NimBLE_Stream_Server example used NUS UUIDs but a single characteristic for both notify (TX) and write (RX), which is non-standard and breaks compatibility with NUS terminal apps (nRF UART, Serial Bluetooth Terminal, etc.). Standard NUS requires separate TX (6E400003, notify) and RX (6E400002, write) characteristics.

Core library changes

NimBLEStreamServer

• Added RxChrCallbacks inner struct to handle writes on a dedicated RX characteristic
• Added m_pRxChr member; single-char mode is fully backward-compatible (m_pRxChr == nullptr)
• New begin(pTxChr, pRxChr, ...) overload for externally-created characteristics
• New begin(svcUuid, txChrUuid, rxChrUuid, ...) convenience overload — creates a proper two-characteristic NUS service internally
• end() restores callbacks and clears m_pRxChr

NimBLEStreamClient

• Added m_pRxChr for the dedicated subscription characteristic
• New begin(pTxChr, pRxChr, ...) overload — pTxChr is written to, pRxChr is subscribed
• end() unsubscribes from the correct characteristic in two-char mode

Example updates

Both NimBLE_Stream_Server and NimBLE_Stream_Client examples updated to use proper NUS two-characteristic layout. NimBLE_Stream_Echo (custom UUIDs, single char) is unchanged.

Server setup now:

bleStream.begin(NimBLEUUID(SERVICE_UUID),
                NimBLEUUID(TX_CHAR_UUID),   // 6E400003 — notify
                NimBLEUUID(RX_CHAR_UUID),   // 6E400002 — write
                1024, 1024, false);

Client setup now:

// pRxChar = 6E400002 (we write here); pTxChar = 6E400003 (we subscribe here)
bleStream.begin(pRxChar, pTxChar);