fix(docs): restore README.adoc as canonical entry point, remove README.md

The 2026-06-12 README dedup pass kept README.md as canonical and
demoted readme.adoc to a one-line pointer, backwards from this
estate's own documented policy (AsciiDoc is canonical; no duplicate
.md when .adoc exists). Worse, the dedup's own heuristic for "which
one is richest" was file size — which picked exactly the wrong file,
since a deliberately thin adoc entry point that correctly delegates
to docs/echo-types/MAP.adoc is good documentation design, not a sign
of being less important.

GitHub's README-picking logic favors exact-case README.md over
lowercase readme.adoc, so GitHub has been rendering the wrong file
as this repo's main page ever since.

Fix: convert the full content (previously in README.md) to proper
AsciiDoc syntax in README.adoc (correct casing), remove both README.md
and the old readme.adoc pointer stub. No content lost — full history
remains in git; this commit's diff is a syntax conversion, not a
content change.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: hyperpolymath