docs(certification): add failure-mode competency scoping document

[bokelley]

Closes #2386

Adds docs/learning/failure-mode-scope.mdx — the scoping document requested in #2386. Maps 12 failure modes to certification modules, depth targets, and assessment approaches. Authoring follow-ups per module are separate issues per the issue's "Done when" criteria.

Non-breaking justification: Purely additive — new documentation file and nav entry, no schema changes, no code changes, no renamed fields.

What's in the scoping document

The original 9 candidate modes from the issue, plus 3 additions from the expert review:

ID	Failure mode	Primary module(s)
FM-1	Idempotency replay / conflict / expired	S1 (already partially covered)
FM-2	Creative compliance failure post-launch	S2 (new lab exercise needed — IACET gap)
FM-3	Payment / settlement reconciliation	S1 (current delivery surfaces; depth TBD pending #2391)
FM-4	Governance token mismatch / revoked mid-lifecycle	S4
FM-5	Lifecycle state stuck	S1, S2, S5, D1/D3
FM-6	Webhook delivery failure / retries	D3 (seller), S1 (buyer)
FM-7	Signed-request validation failure	D2 (implementation), S1 (reasoning)
FM-8	TMP provider integration failure (reclassified — see below)	S1, D3
FM-9	Cross-protocol policy conflicts	S4
FM-A	ACCOUNT_PAYMENT_REQUIRED blocking active buys	S1
FM-B	report_usage pricing mismatch	S1
FM-C	adagents.json / brand.json authorization failure	D2, S1, S4

FM-8 reclassification: The issue listed "TMP attestation failure." The document reclassifies this to "TMP provider integration failure" (adagents.json binding, seller_agent URL mismatch, bypass-mode misconfiguration) because TMP cryptographic attestation is marked SHOULD/future-enhancement in AdCP 3.0 — teaching it now would credential knowledge of an unstable feature rather than current protocol behavior. Cryptographic attestation scenarios are deferred until TMP exits experimental.

Nits noted (not fixed — authoring time)

• Criterion IDs are specified for FM-3 (s1_recon_sc0) and FM-7 (d2_sig_sc0, s1_sig_sc0) where the recertification risk is highest; all others are deferred to authoring issues with an explicit note to assign IDs.
• FM-8 deferred scope needs a tracking hook — a one-liner pointing to wherever TMP attestation work lives should be added by the author closing the FM-8 authoring issue.

Pre-PR review:

• code-reviewer: approved — 1 blocker (missing changeset, fixed), 3 issues (non-standard depth label, informal depth variant, JSX-unsafe bracket sequence in FM-9 exam question, all fixed), 2 nits (description quotes consistency fixed; pre-existing docs.json gap in latest Governance group is out of scope for this PR)
• education-expert: approved — 2 blockers fixed (Apply label standardized, FM-2 IACET gap note extended with authoring spec), 2 nits noted (criterion ID consistency, FM-8 tracking hook — deferred to authoring)

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See #3121
for context.

Session: https://claude.ai/code/session_011hn2S5768vHMSTL9YZmEAJ

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