Introduce WebSDR backend enabled via ENABLE_WEBSDR, adding an alternative callback-based transport alongside existing socket-based TRX/OSMUX I/O
Implement: - TRX control via osmotrxlib_process_command() - TRXD data via apitrx_tx_call()/trx_data_read() - clock handling via apibts_clock_ind() and external timers - Osmux integration via ws_osmux_* helpers
Adapt scheduler to support API-based burst submission and external timer callbacks in WebSDR mode
Disable signal handling, stats and socket I/O when building with ENABLE_WEBSDR
Add WebSDR API entrypoint, Emscripten build target and example configuration
Enables running BTS/TRX stack in WebAssembly environments