docs: add N'KEMBA institutional reconstruction crosswalk

[riscas028]

Summary

Adds a non-normative crosswalk for N'KEMBA as a downstream institutional-reconstruction consumer of TRACE evidence.

The crosswalk:

• keeps TRACE references entries as pointers rather than attested evidence;
• separates runtime verification from authorization, operative-document, external-outcome, and adoption layers;
• states both what each input can satisfy and what it does not satisfy by itself;
• describes fail-closed consumer outcomes without adding fields or conformance requirements to TRACE;
• uses only the registered authorized-intent, approval-outcome, and behavior-trace relationship values;
• adds the page to the Crosswalks navigation.

Scope

Documentation only. No schema, wire-format, required-claim, registry, or conformance change.

Review context

Follows the venue and boundary guidance in #274 and the structure of the Acta crosswalk.

The contributor has reviewed the proposed text and can explain its intended mapping and boundaries.

Closes #274