spec(compliance): force_scenario_unsupported — UNKNOWN_SCENARIO on force_* steps grades not_applicable

[bokelley]

Closes #4226

Summary

The compliance runner graded create_media_buy_async as FAILED when a seller returned UNKNOWN_SCENARIO on force_create_media_buy_arm. The storyboard narrative already said this should grade not_applicable, but the runner contract had no machine-readable enforcement layer for explicit force_* phase steps — only fixture_seed_unsupported (auto-injected fixture phase) was documented.

This PR adds the missing spec text so conforming runners can implement the rule.

Changed files:

• static/compliance/source/universal/storyboard-schema.yaml — adds force_scenario_unsupported block with normative MUST for runners: detect the tuple (comply_test_controller present, resolved payload scenario begins with force_, response {success: false, error: UNKNOWN_SCENARIO}) and grade not_applicable before evaluating declared validations. Documents detection order to prevent misgrading absent-tool cases.
• static/compliance/source/universal/runner-output-contract.yaml — adds fixture_seed_unsupported, force_scenario_unsupported, and unresolved_scenario_reference as recognized narrower detail values under canonical reason: not_applicable, with the encoding MUST for force_scenario_unsupported.

No storyboard YAML changes — create_media_buy_async.yaml's narrative was already normatively correct.

Non-breaking justification

Patch-eligible per the conformance-harness carve-out and the IETF errata test (playbook lines 261-265): the storyboard's own narrative already required not_applicable grading; runners emitting FAILED were non-conforming against existing normative English. This adds the machine-readable form of a rule already in force — not a new requirement. The canonical reason enum in runner-output-contract.yaml is unchanged; force_scenario_unsupported is a detail sub-value under the existing not_applicable reason.

Sibling-repo fix needed

adcp-client runner needs implementation to honor this contract. File: the UNKNOWN_SCENARIO detection path in the storyboard runner (see src/lib/testing/storyboard/ and DETAILED_SKIP_TO_CANONICAL mapping from PR #611). The detection order MUST place force_scenario_unsupported check after confirming comply_test_controller is present (to avoid misgrading the absent-tool case).

Pre-PR review

• code-reviewer: approved — no blockers; nit on encoding MUST placement addressed in final diff
• ad-tech-protocol-expert: approved — non-breaking per spec; nit on resolved-payload wording and detection-order addressed in final diff

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See #3121
for context.

Session: https://claude.ai/code/session_01Uu4K4voWNzFUCUQzKyrsiC

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