docs(architecture): strip Plan-9/superpowers refs from HTML + wire orphan explainers

Closes the last two staleness threads from the docs audit:

1. data-flow-explainer.html had four "Plan-9" / "post-Plan-9"
   references in the masthead, lede, fix-up bullet, and bindDevSeed
   blurb. Stripped — the architecture is now just "vertical-feature".
   No content changes beyond the noun rename.

2. audit-and-compliance-explainer.html had a footer link to
   ../superpowers/specs/2026-05-11-audit-and-compliance-design.md
   (archived to .archive/ earlier). Replaced with a link to
   ADR-018, which is the durable design record.

3. data-flow-explainer.html + di-explainer.html were inter-linked
   with audit + conformance explainers, but had no markdown entry
   point — they were orphans from any guide or architecture doc.
   architecture/overview.md gains a new "Interactive explainers"
   section listing all four single-file HTML walkthroughs with one
   sentence each, so they're discoverable from the documented
   entry point. The four pages already cross-link to each other.

Final state (verified by repo-wide grep): zero "Plan-N" / "Phase-N"
/ "docs/superpowers/" references in docs/ (excluding .archive/ which
is gitignored). Legitimate `superpowers:brainstorming` skill refs
in agent-first-workflow-and-conformance.md are preserved — those
reference an external plugin, not a repo path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/architecture/audit-and-compliance-explainer.html

@@ -1517,7 +1517,7 @@ footer .colophon {
       <a href="vertical-feature-spec.md">vertical-feature-spec.md</a>
       <a href="overview.md">overview.md</a>
       <a href="dependency-flow.md">dependency-flow.md</a>
-      <a href="../superpowers/specs/2026-05-11-audit-and-compliance-design.md">2026-05-11 · audit design spec</a>
+      <a href="../decisions/adr-018-audit-and-compliance.md">ADR-018 · audit &amp; compliance</a>
     </div>
     <div>
       <h5>Guide</h5>