Protocol: declare a chain of instances run in one worker, with crash quarantine (for omx-clap-core#104) #101

What is missing. The protocol has no way to say "these instances are one chain, in this order". omx-clap-core#104 makes omx-clap-host run a chain of plugins in one JACK client (one graph node, one wake-up per cycle instead of one per plugin), and it needs plugin-hostd to declare the chain and to route it to one worker.

Ruled 2026-10-19 (by recommendation, coordinator decision log row 00:55, omx-clap-core#104). The protocol gets a chain declaration that lists instances in order. A member that crashes is quarantined to a worker of its own, and the chain rejoins around it: crash isolation is plugin-hostd's purpose, and one plugin must never take its neighbours down.

Where it plugs in. protocol/plugin-hostd.json and its schema, include/plugin-hostd/protocol.h (generated, check-generated), src/verbs.c and src/supervisor.c (one worker per chain; on a member's crash, restart the chain without it and give the crashed member a worker of its own), tests/verbs_contract.py, and a tests/daemon_test.py scenario with the fake host.

Done when.

• A chain verb declares an ordered list of existing instances. An unknown instance, a duplicate, or an instance already in another chain is refused with a named code. Removing a chain gives each member back its own worker.
• With the fake host, a chain of three is started as one worker. When the middle member is made to crash, the chain restarts with the other two in order, the crashed one runs alone in a worker of its own, and an event names the quarantine.
• Bypass, latency and parameter verbs still address each instance by its own id inside a chain.
• Every guard has been broken once and went red (make sabotage).

Blocks omx-clap-core#104. Part of FreeMixer/openmixer#2041 (plugin hosts: full LV2 and CLAP support), P2.

PRIORITY + BASE 2026-10-21 (operator: multiple plugins in one process is a selling point)

P1, start now. BASE: branch off origin/wave/contract-one-pass (the contract one-pass wave, J2 H1-H5 are on it), NOT main; open the PR into wave/contract-one-pass. Do not touch the tables or describe verb J2 added; the chain verb is new. Commit after every working step. Unblocks omx-clap-core#104.

SCOPE ADDITION 2026-10-21 (Zynthian + console need live chain edits; from the chain-model comparison)

Done-when additions, to be judged at hand-back (the r2 build branch cluster/plugin-hostd-101-r2-cluster was briefed before this):

• Adding a member to, or moving a member within, a running chain is a LIVE edit (no fresh worker for the surviving members); removing already is.
• A chain read-back verb: members in order, and which are quarantined; worker_list lists a chain's members.
• A worker that does not know the chain line is NOT killed (supervisor.c on_death on a non-RPC_OK reply: confirm).
• Zynthian: one Zynthian chain maps to one chain here (ordered members, per-member bypass/params). The console maps each trip (a run of adjacent isolated plugins) to one chain.
  If the build branch lacks these, they become a follow-up lane on this issue, not a reason to restart.