MT5700 AT transport fix
- Remove the dial handler's dependency on the external timeout utility. Use the shared AT transport with an eight-second command watchdog and a five-second port-lock wait. Previously, a missing timeout utility prevented AT commands from being sent while hiding the launch error as an apparent registration failure.
- Distinguish missing or unparseable registration replies from an explicitly unregistered state. The retry log identifies the selected PCUI port, and exhausted startup reports TDTECH_REGISTRATION_UNAVAILABLE when registration could not be read.
- Preserve the existing USB autodial and DHCP sequence and working CDC NCM bindings.
The latest issue #68 report confirms that CDC NCM creates the network interface and the modem answers registration queries on PCUI. The missing-utility failure is reproduced locally; the report does not directly establish whether timeout is installed on that router.
Validation: 60 regression tests and all 17 MT5700 profile scenarios passed. Tests cover operation without a functional timeout utility, busy-port refusal, termination of a silent AT process, lock release and parsing the reporter's registered state. Connection and traffic on the reporter's modem still require hardware confirmation.