feat(ios): W2 connection core — URLSessionTermTransport + SessionEngine actor
T-iOS-9: TermTransport impl (Origin single-source, 16MiB from Tunables, re-arm loop,
EMSGSIZE(40)/ENOBUFS(55)→typed replayTooLarge, delegate-driven state, RFC6455 scripted test server), 11 tests
T-iOS-10: SessionEngine actor (attach-first ordering, generation guard, terminal states never backoff,
exactly-once digest, gate canDecide second line, defensive input validation), 20 tests
Verified: 209 tests green across 5 packages; SessionCore own-src coverage 96.68%; 4/4 semantic invariants
audited in source; frozen contracts untouched
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