Replace cssl_lib with serialclass and eliminate cssl_lib

[ejpilger]

cssl_lib (C-style serial API, micro-agent layer) and serialclass (C++ Serial class, kernel layer) cover the same functionality. The APIs map 1-to-1:

cssl_lib	serialclass
cssl_open()	Serial(name, baud, …) + open_device()
cssl_close()	close_device()
cssl_setup()	set_params(), change_baud()
cssl_setflowcontrol()	set_flowcontrol()
cssl_settimeout()	set_timeout()
cssl_putchar/string/data/slip/nmea()	put_char/string/data/slip/nmea()
cssl_getchar/data/slip/nmea/xmodem()	get_char/data/slip/nmea/xmodem()
cssl_drain()	drain()
cssl_geterror()	get_error()

serialclass is also richer, adding set_dtr(), set_rts(), get_cts(), poll_char().

Consumers to migrate (all in device libs):

• arduino_lib (micro-agent)
• gs232b_lib, kisstnc_lib, kpc9612p_lib, mixwtnc_lib, pic_lib, ts2000_lib (ground-station)
• bbFctns — cssl_lib already commented out, no change needed

Benefit: serialclass lives in kernel (Layer 0). After migration, these device libs depend on kernel directly rather than micro-agent. cssl_lib and the CosmosCssl CMake target can be removed entirely.

Approach: For each consumer, replace cssl_t* handle and cssl_* function calls with a Cosmos::Serial member and the equivalent method calls. The refactoring is mechanical.