Closes the design+build for brandon-layout-substrate-2de.4 — the feature layer atop the container|segment + actions foundation. Two additions, zero render-engine work; everything desugars to the shipped grammar.

1. cycle action arm (the enumerated-domain stepper)

{ set: key, cycle: [v0, v1, …] } writes the successor of the current value, wrapping; a current value outside the domain counts as the first member (order members default-state-first). The gate derives as an allow-list of the members through the existing same-key merge — no validator plumbing, the vocabulary grows by arms.

{{ action }} grows a variadic display form for cycle actions: one display per member, positionally matched — {{ action "toggle" "▸ details" "▾ details" }} — or one static display. The current member's display renders; the click writes the successor. Non-cycle kinds keep display [boundValue]; wrong arity is a loud render error.

Deliberate contrast with set-bounded: a cycle emits the absolute successor computed at render (the glyph names the current state, so the click delivers exactly the transition the glyph promised — [LAW:one-source-of-truth], display and write derive from one read), where the bounded stepper emits a relative nudge (rapid clicks must accumulate).

2. group layout sugar (input-only node kind)

{ kind: "group", name, label, open?, direction?, key?, bg?, fg?, when?, children } lowers at the loader to canonical container/segment nodes and synthesizes its state var + cycle action + toggle segment under the reserved groups. namespace — one declaration, every derived artifact single-sourced ([LAW:one-source-of-truth]; a user name under the prefix is a load error). Group is never a canonical LayoutNode kind ([LAW:one-type-per-behavior] — arranging + gating are behaviors container already has).

• Independent toggle: omit key.
• Accordion: siblings share a key — one key holds one open name, so opening a sibling auto-closes the rest. Not a mode, a shared value ([LAW:dataflow-not-control-flow]); the gate is the union of the sibling cycles via the existing merge. No path grammar, no new action arm needed — the accordion semantics fall out of cycle's "unknown current counts as the first member" rule.
• Nested disclosure: nested groups with distinct keys; a closed parent's when hides the subtree, child state persists invisibly. Ancestor/descendant sharing a key is a load error.

Verification

• 32 new tests through the real spine + real daemon verb handlers (toggle round trip, accordion auto-close, nested persistence, every loader invariant), schema GOOD/BAD parity cases; full suite 1171/1171.
• Live-path verified on the built dist with an isolated daemon: rendered closed toggles → url-handle click → ▾ + body → clicking the sibling auto-closed the first. Real OSC-8 wire URLs through the real set-state gate.
• Schema regenerated from the loader declarations (check:schema green); check:protocol green.
• Folds the pending CLAUDE.md merge-vs-replace doc-drift fix.

Out of scope (by design review with the user): auto-indent for nested groups → brandon-layout-polish-atn; a {{ toggle }} helper — subsumed by the variadic display form.