nx_connect_groupkey.nx

nx_connect_groupkey.nx -- VIDEO ROOM E2E group keying (MLS-style sender-key ratchet). Hardens the content-blind exceed from "server-decoded=0 by design" to "cryptographically cannot decode, over time and across membership changes." Models the key SCHEDULE (integer one-way KDF); the production primitive is the ecosystem HKDF/SHA -- named substrate, not reinvented here. Key schedule: ratchet(k) = KDF(k + RATCHET_LABEL) -- per-epoch forward ratchet (forward secrecy) rekey(k, commit) = KDF(k + commit) -- membership change; commit secret goes ONLY to current members, so a removed member diverges Properties gated: (1) forward secrecy -- a current key cannot derive a PAST key (one-way); (2) removed member cannot derive the post-removal key (no commit) -> excluded from future; (3) added member gets current+forward only, never the past; (4) the server holds ZERO key material (reinforces content-blind); (5) membership change increments epoch + rotates the key; (6) NEG-CONTROL: a static single key exposes ALL epochs on one compromise; our ratchet exposes 0 past epochs. 7 checks. 100% sovereign. license_tier: ORIGINAL expect_exit: 0