feat(society): N-way authenticated field-coupling swarm plane (societies epic, increment 1)

[NickFlach]

First increment of the N>2 field-societies epic: the ADR-0019 group session key now gates the FSYN field-coupling wire at N=3 (previously keyed operation was only ever tested at 2 VMs; boot_n had no key path).

The panel's central correction (why the naive gate was unshippable)

A 3-lens adversarial review established — confirmed on fieldsyncd.c — that an R_x / SYNCHRONIZED assertion cannot detect a keyless node. A keyed receiver rejects zero-tag frames, but an unkeyed receiver accepts everything (including keyed frames) and becomes a one-way Kuramoto follower, phase-locking to the keyed pair and printing SYNCHRONIZED anyway. So the gate discriminates purely on frame admission: the FSAUTH bad-MAC reject line + a per-source FIELDSYNC: frame from <ip> growth differential, offset-anchored at each member's FSKEY position.

Increment 0 — guest observability (user/fieldsyncd.c)

• note_reject fires once-per-reason (bad-MAC / replay latches) — a boot-global latch swallowed the replay line behind the exclusion's bad-MAC line.
• note_key prints on re-install (FSKEY: rekeyed…) — a rekey was console-silent.
• Resolved the ADR-0019 n>=296 vs ==296 doc/code drift (kept >=; the MAC covers the fixed prefix). STATUS_BODY not widened — it's frozen ABI (contracts/wire/v1.golden).

Increment 1 — host + gate

• QosSociety.admit_key_n(key, indices): the same group key to each member's attested COM2 (per-member admit_key, never attest() — which mints a random per-VM key that would make the society mutually reject).
• since= offset anchoring through the status helpers (the sticky whole-log grep reads stale cross-phase evidence); frame_from_count for the admission differential.
• boot_n randomizes the mcast group per boot + scales the precheck deadline with N.
• test_qos_keyauth_n.py (3 VMs): leg (a) exclusion, leg (b) admit-and-resync, leg (c) a real host mcast capture-replay (stdlib join, verbatim re-send → valid MAC + stale seq → per-reason replay reject). make ci-smoke-keyauth-n-gate; new society-n CI job.

Verification (local WSL)

--- leg (a): EXCLUSION — key A,B only, C withheld ---
OK: A and B both reject the keyless peer's forged-MAC frames
OK: neither A nor B admits ANY frame from the keyless C (delta==0)
OK: keyless C still receives fresh frames from A(14) and B(14) — the one-way lock R_x cannot discriminate
--- leg (b): ADMIT ---   OK: coupling resumed; all three synchronized (0.99)
--- leg (c): REPLAY (real host injector) ---   OK: member rejected the replayed frame (valid MAC + stale seq)
=== N-WAY KEYAUTH gate PASSED ===

Revert-confirmed both ways (skip leg-b admit → red; collapse the per-reason flag → replay marker vanishes → red). society3 + 2-VM keyauth regressions still pass; kernel builds.

Increments 2 (the N=4 configuration) and 3 (N-way society-of-societies) follow.

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