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/overview.md

@@ -133,3 +133,14 @@ Each feature owns its own InversifyJS `Container` + symbol table. No shared symb
 ## Spec reference
 
 `docs/architecture/vertical-feature-spec.md` is the canonical design.
+
+## Interactive explainers
+
+Four single-file HTML walkthroughs sit alongside this overview. They're self-contained (no build step) and best viewed locally in a browser.
+
+- [`data-flow-explainer.html`](./data-flow-explainer.html) — request and event flow through one feature, with the DI binding mode toggle and feature swap controls.
+- [`di-explainer.html`](./di-explainer.html) — the full container + symbols + binder lifecycle.
+- [`feature-conformance-explainer.html`](./feature-conformance-explainer.html) — companion to ADR-020-adjacent conformance design (also linked from `agent-first-workflow-and-conformance.md` and `runbook.md`).
+- [`audit-and-compliance-explainer.html`](./audit-and-compliance-explainer.html) — companion to ADR-018 (also linked from `dependency-flow.md` and `guides/audit-and-compliance.md`).
+
+The four pages cross-link to each other; entering any of them surfaces the others.