Unit-testable browser WebSocket client: inject WS + timers, cid/ack request()/promise, full-jitter backoff

Building a resilient WS client for a browser SPA that must ALSO be importable/unit-testable under node (no jsdom):

1) Never reference WebSocket/setTimeout/document/localStorage at module top level. Look them up lazily and allow injection: createWsClient({WebSocketImpl, setTimeout, clearTimeout}). Tests pass a MockWS class recording sent frames + driving _open/_recv/_drop, and a manual timer scheduler (map of id->{fn,delay}, runNext() returns the delay) so exponential backoff is asserted deterministically WITHOUT fake timers libs.

2) request(t,payload) -> promise resolved by matching ack: auto-increment a cid, store {resolve,reject} in a Map keyed by cid, and in the onmessage dispatcher check msg.cid: if pending, resolve on any type or reject when t===error (attach msg.code). Also reject ALL pending on socket close so callers never hang.

3) Backoff gotcha that bit me in a test: a SUCCESSFUL onopen must reset attempts=0, so backoff restarts at base after any good connection. My first test wrongly expected 500->1000 across a drop that had an _open() in between. Correct: only consecutive FAILED reconnects (open->drop before onopen) grow the delay. Full jitter = raw/2 + rand*raw/2.

4) On reconnect, re-send a resume{gameId,lastAppliedSeq}; track lastAppliedSeq from inbound ack.seq and moved.n so resume asks for the right point.

Same injection pattern makes a REST client testable: configure({fetch}), read CSRF from a non-HttpOnly cookie only for mutating verbs, normalize errors to {code,message,status} and map network failure to a stable code=offline so the UI can fall back to an offline path instead of crashing.