refactor: migrate Markdown to AsciiDoc (#366)

Automated migration of .md to .adoc per global rules, preserving wikis
and critical exceptions.

Author: hyperpolymath

ARCHITECTURE.adoc

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+== Architecture
+
+The architecture documentation for ECHIDNA lives in
+link:docs/ARCHITECTURE.md[`+docs/ARCHITECTURE.md+`] — the polyglot
+layout, the dispatch path, the trust-hardening pipeline, and how the
+Rust core, Julia ML sidecar, Idris2 ABI, Zig FFI and optional Chapel
+parallel layer fit together.
+
+Related canonical documents:
+
+* link:docs/PROVER_COUNT.md[`+docs/PROVER_COUNT.md+`] — backend tier
+table and what each published count actually counts
+* link:docs/ROADMAP.md[`+docs/ROADMAP.md+`] — stage map and current
+direction
+* link:docs/DEBT.md[`+docs/DEBT.md+`] — known licence, documentation and
+code debt
+* link:.machine_readable/descriptiles/META.a2ml[`+.machine_readable/descriptiles/META.a2ml+`]
+— the machine-readable architecture record

ARCHITECTURE.md

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+== Authors and Contributors
+
+This file lists the authors and contributors to the ECHIDNA project. The
+authoritative running list lives in the git history —
+`+git shortlog -sne+` will produce a deduplicated, frequency-sorted view
+at any commit.
+
+=== Maintainers
+
+[width="100%",cols="34%,33%,33%",options="header",]
+|===
+|Name |Role |Contact
+|Jonathan D.A. Jewell |Lead Maintainer
+|https://github.com/hyperpolymath[@hyperpolymath],
+`+j.d.a.jewell@open.ac.uk+`
+|===
+
+See link:MAINTAINERS.adoc[`+MAINTAINERS.adoc+`] for the maintainer
+responsibility model and the path to becoming a maintainer.
+
+=== Contributors
+
+Code, documentation, infrastructure, review, issue triage. The
+deduplicated running list is in the git history:
+
+[source,bash]
+----
+git shortlog -sne
+----
+
+We recognise all forms of contribution including but not limited to:
+
+* Code contributions
+* Documentation improvements
+* Bug reports and testing
+* Feature suggestions
+* Community support
+* Translations
+* Design and artwork
+* Infrastructure and tooling
+
+=== Special Thanks
+
+* *The theorem-proving community* — for the foundations and the ongoing
+maintenance of the backends ECHIDNA integrates with (Coq/Rocq, Lean,
+Agda, Isabelle, Idris2, F*, HOL Light, Mizar, PVS, ACL2, HOL4, Z3, CVC5,
+Vampire, E Prover, and the long tail in
+link:docs/PROVER_COUNT.md[`+docs/PROVER_COUNT.md+`]).
+* *The Rhodium Standard Repository (RSR/CCCP)* — for the compliance
+template that this repo extends; see
+link:RSR_COMPLIANCE.adoc[`+RSR_COMPLIANCE.adoc+`].
+
+=== How to be added
+
+If you’ve contributed to ECHIDNA and would like to be acknowledged
+beyond the git history, open an issue or PR on
+https://github.com/hyperpolymath/echidna/issues[GitHub] with the
+attribution you would like.
+
+=== License of contributions
+
+Source files are licensed per the SPDX header at the top of each file;
+the project root link:LICENSE[`+LICENSE+`] and the per-tree
+link:.reuse/dep5[`+.reuse/dep5+`] catalogue together describe the
+authoritative position. By contributing you agree to license your
+contribution under the licence noted on the file you touch (or the
+project default for new files).
+
+The documentation surface is intentionally `+MPL-2.0+` — see
+https://github.com/hyperpolymath/echidna/issues?q=license[`+feedback_echidna_license_docs_mpl_intentional+`]
+for the deliberation.
+
+=== Contact
+
+* *GitHub Issues*: https://github.com/hyperpolymath/echidna/issues
+* *Security*: see link:SECURITY.md[`+SECURITY.md+`] — use GitHub
+Security Advisories for vulnerabilities, not public issues.
+
+'''''
+
+_This file is intentionally count-free and date-free in prose. The git
+history is the running timeline; the canonical attribution surface is
+`+git shortlog -sne+`._