docs: add architecture explanation page

[STRRL]

Problem

The architecture write-up (data flow, DNS management, connector reconciliation loop) lives only in CLAUDE.md and is not published on the docs site. The Explanation quadrant of the Divio documentation system is essentially empty. The docs site also has no Mermaid rendering support, which the new diagram-first documentation style requires.

Part of #334.

Solution

Publish an architecture explanation page distilled from CLAUDE.md and verified against the actual sources, point CLAUDE.md at the published page, and add site-wide Mermaid rendering via the astro-mermaid integration.

Major Changes

• docs/src/content/docs/explanation/architecture.md (new)
  • Two focused Mermaid diagrams: the data flow (IngressController watches Ingress, transforms to Exposure, TunnelClient updates tunnel ingress rules, cloudflared routes traffic) and the DNS model (proxied CNAME to <tunnel-id>.cfargotunnel.com, _ctic_managed.<hostname> TXT ownership records, disable-dns-management semantics), plus prose on the 10s ControlledCloudflaredConnector reconciliation loop.
• Mermaid infrastructure (site-wide)
  • astro-mermaid integration added to docs/astro.config.mjs and docs/package.json; any page's ```mermaid fence now renders as a diagram. The diagrams in the troubleshooting and how-to PRs depend on this, so merge this PR first.
• CLAUDE.md
  • Architecture section slimmed to component names plus file paths, pointing at the published page.

Note: the sidebar entry for the new Explanation group is added in the upcoming sidebar reorganization PR; until then the page is reachable by direct URL.

Docs were written by Codex (gpt-5.6-sol) and reviewed for source accuracy by a separate review agent.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.38%. Comparing base (5995da2) to head (1de80de).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #335   +/-   ##
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  Coverage   53.38%   53.38%           
=======================================
  Files          14       14           
  Lines         871      871           
=======================================
  Hits          465      465           
  Misses        376      376           
  Partials       30       30           

Flag	Coverage Δ
integration	27.58% <ø> (ø)
unit	40.16% <ø> (ø)

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