Wire up the previously-dead Flipper Zero handshake/SPI code from
at32_unit_test.c into a real WITH_CEP platform extra, independent of
BWM (new g_reply_via_cep flag, not reusing BWM's g_reply_via_fpc, so
both transports can run concurrently).
Fixes a real reconnect bug found during hardware bring-up: the
handshake listener was gated on a CC-controller attach edge that
didn't reliably fire on every FAP relaunch, requiring a physical
cable replug to reconnect. Now watches for a fresh incoming byte any
time the link reports attached, with a bounded receive timeout so a
stray byte can never hang the main loop.
RF/FPGA-dependent functionality (hf/lf reads) is out of scope here -
tracked separately. Closes#3667.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>