Add test/web-protocol.test.js — runs the real protocol code from chat.html in
Node (unit: crc/frame/ACK/HELLO codec, Hamming, Gray; integration: multi-level
modem roundtrip + FEC recovery of off-by-one symbol errors; functional: full
frame -> modem -> FEC -> assembler -> parse + ACK roundtrip). 3348 assertions.
Wired as `make test-web` (also in test-all). Lets us QA the modem without two
devices. Also: retransmit now uses exponential backoff so a lost ACK spaces out
retries instead of hammering the channel.
