Changes

trx, bts: add optional WebSDR backend (callback-based transport)

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

Change-Id: Idebd588f33afd85021813ad5821391781db683a9