docs(compliance): add compliance grading model reference page

[bokelley]

Closes #4036

Adds docs/building/verification/grading-model.mdx — a single reference page explaining the AdCP compliance grading model end-to-end. The page is placed in the "Verification & trust" nav group, after "Validate Your Agent".

Motivation: An SDK contributor (implementing seller-side framework in adcp-client-python) spent over an hour misreading the system — looking at JS test helpers (src/lib/testing/scenarios/media-buy.ts) instead of the authoritative YAML storyboards. This caused an incorrect architectural claim that the proposal lifecycle wasn't being exercised when it was. A single doc page closes that discoverability gap.

What's covered:

1. Specialism declaration — specialisms field in get_adcp_capabilities, kebab-case enum IDs, parent-protocol requirement
2. Scenario resolution — three-layer taxonomy (Universal → Protocol → Specialism), two-phase merge of protocol baseline + specialism requires_scenarios, deduplication and capability-gate application; explicit callout that the YAML storyboard runner is authoritative, not the JS test helpers
3. Capability gates — requires_capability YAML block, capability_unsupported skip semantics with --json output example sourced from proposal_finalize.yaml; "(or omit)" default behavior sourced directly from sales-guaranteed/index.yaml:49
4. Reading results — accurate overall_status values (passing/failing/partial from compliance-db.ts:13), tracks_passed, steps_passed/steps_total, storyboard_id; debug CLI examples
5. Invariants — status.monotonic as a separate failure axis from step-level validations, with pointer to invariant_failures in --json output

Non-breaking justification: New MDX page added to existing nav group; no existing content removed or modified. Zero wire impact.

Pre-PR review:

• code-reviewer: approved — blocker (wrong storyboard ID in CLI example) fixed; all CLI examples now use valid storyboard IDs consistent with validate-your-agent.mdx
• docs-expert: approved — dual-audience framing works; three-layer resolution table and requires_scenarios YAML match source files; related links non-orphaned

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_01CPn4oJdWTTVZ2L8WCD4pDg

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