docs: add N’KEMBA institutional reconstruction cross-walk

[riscas028]

Adds a non-normative downstream-consumer cross-walk for N’KEMBA, following the venue and scope guidance in #274.

The cross-walk:

• leaves TRACE v0.2 schema, wire format, claims, verification rules and conformance requirements unchanged;
• treats TRACE evidence as upstream evidence rather than institutional proof;
• preserves the §3.1.2 references boundary: a reference is a pointer, not attested truth about its target;
• separates runtime/session evidence, action evidence, human authority, operative document/version, external outcome, and institutional adoption;
• states both what the mapping covers and what N’KEMBA does not claim;
• adds the corresponding Crosswalks navigation entry in mkdocs.yml.

The N’KEMBA demonstrator mentioned in the document is explicitly described as a synthetic fail-closed functional-logic demonstrator, not TRACE conformance, certification, endorsement, or independent audit. The v0.7 default-pass issue discussed in agentrust-io/trace-tests#96 was corrected before this contribution was prepared.

Related: #223, #274; agentrust-io/trace-tests#96.

[riscas028]

Maintainer-facing note following the latest verifier/reference discussions: this cross-walk intentionally treats two boundaries as downstream hand-off points rather than TRACE claims. First, an unresolved or insufficiently established external fact remains explicitly unproved downstream; it is not promoted to success or converted into TRACE verification failure. Second, resolving a references URI does not by itself establish object identity, content immutability, authority, or institutional status; where a digest/binding is available N’KEMBA preserves it, and where it is absent the downstream conclusion remains bounded accordingly. This is consistent with the PR’s existing non-normative scope and does not propose a TRACE schema change. If maintainers prefer, I can fold this clarification into the cross-walk text during review rather than add normative language.