docs(compliance): document branch-set any_of peer cascade exemption

[fgranata]

What

Adds a named branch_set_cascade_exemption to cascade_rules in static/compliance/source/universal/runner-output-contract.yaml, parallel to the existing sole_stateful_step_exemption:

When two or more phases share the same branch_set.id with any_of semantics, a stateful peer's genuine failure or peer_branch_taken skip MUST NOT cascade prerequisite_failed onto a sibling phase sharing that branch_set.id. Each peer is graded on its own merits, and the any_of gate credits whichever peer the agent satisfied.

Also:

• default_cascade gains a one-sentence pointer to the new exemption (so it is not silently contradicted by the existing "peer_branch_taken MUST cascade" text).
• storyboard-schema.yaml's depends_on section gains a cross-reference.
• A patch changeset documenting the clarification.

Why

any_of branch-set peers are mutually-exclusive alternatives, not a dependency chain. A conformant agent satisfies exactly one peer, so the non-taken peer(s) fail by design. Under the literal default_cascade reading (which lists peer_branch_taken and genuine failures as cascade triggers), an earlier peer's by-design non-selection was cascading prerequisite_failed onto a later sibling the agent actually satisfied, returning an empty any_of gate for a conformant agent (surfaced on media_buy_seller/refine_finalize_exclusivity for a seller that correctly rejects atomic multi-finalize with MULTI_FINALIZE_UNSUPPORTED).

Scope / properties (all confirmed against the shipped runner)

• any_of only — other (future) branch-set semantics where peers legitimately depend on each other keep cascading.
• depends_on-agnostic — fires whenever two phases share an any_of branch_set.id, whether the dependency is the implicit default or an explicit depends_on. depends_on answers whether an edge exists; the cascade exemptions answer how a skip is classified — orthogonal, and neither existing exemption inspects depends_on.
• N-ary-safe — one or more peer failures exempt every sibling in the set.
• Cross-set and within-phase cascade unchanged.

Nature

Documents-only. This codifies runner behavior already shipped in adcontextprotocol/adcp-client#2306 (closing adcontextprotocol/adcp-client#2305), root-caused in #5337. No schema or wire change. The exemption text mirrors the structure and reference style of sole_stateful_step_exemption.

Root-cause investigation, the N-ary confirmation, and the depends_on-agnostic determination are in the #5337 thread.

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[fgranata]

@bokelley gentle nudge on this one — it's the contract PR you green-lit in #5337 (comment) ("Go ahead with the contract PR"), scoped exactly to the implementation notes there: cascade_rules.branch_set_cascade_exemption in runner-output-contract.yaml with the depends_on-agnostic statement, the one-line cross-ref in storyboard-schema.yaml, and a patch changeset documenting the behavior that already shipped in the runner. All checks green. Once it lands, #5337 can close out fully.