BLE write-size probe

Sweep payload sizes through both write APIs and verify per the peripheral's echo

Companion to issue 40686244. The historical Windows behavior:

• writeValueWithoutResponse() rejected payloads > 20 bytes.
• writeValueWithResponse() accepted payloads > 20 bytes.

This page writes a deterministic payload at each requested size via both APIs, then waits for the firmware's ASCII RX seq=N len=L first=0xXX last=0xYY notification to confirm the full payload arrived. Per-cell verdict:

• OK -- JS Promise resolved AND device reported the same length AND first/last bytes match.
• PARTIAL -- write resolved but device received a shorter payload (silent truncation).
• FAIL -- JS Promise rejected, or no echo arrived.

--enable-features=NewBLEGattSessionHandling,WebBluetooth

The flag is needed for getMTU(). The write-size behavior under test is unrelated to the flag.