docs: add ToCs + clear obsolete SPDX-normalization follow-up note (#151)

## Summary

Three small docs follow-ups bundled into one PR:

- **`README.md`** — adds a manual markdown Contents section after the
badges block, mirroring the Axiom.jl-style top-of-file ToC pattern
- **`readme.adoc`** — adds `:toc:` + `:toclevels: 2` macros
- **`EXPLAINME.adoc`** — adds `:toc:` + `:toclevels: 2` macros (was
missing — the other audience-facing `.adoc` files already had `:toc:`)
- **`README.md`** + **`readme.adoc`** — drop the "current practical
follow-up" licensing bullet (the SPDX-header normalization it referenced
just landed in #150, so the line is now obsolete) and tighten the
remaining bullets from "intended to be licensed" → "licensed"

`QUICKSTART-DEV.adoc` / `QUICKSTART-MAINTAINER.adoc` /
`QUICKSTART-USER.adoc` / `INDEX.adoc` / `TOPOLOGY.adoc` already carry
`:toc:` so are unchanged.

## Test plan

- [ ] CI passes
- [ ] `README.md` Contents links resolve correctly on GitHub
- [ ] `readme.adoc` + `EXPLAINME.adoc` ToCs render in any AsciiDoc
viewer

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: hyperpolymath

EXPLAINME.adoc

@@ -1,6 +1,8 @@
 // SPDX-License-Identifier: CC-BY-4.0
 = echo-types: explain me
 :icons: font
+:toc:
+:toclevels: 2
 
 == One paragraph
 

README.md

@@ -15,6 +15,27 @@ Constructive Agda development for echo types as a first-class notion of structur
 
 loss that is not total erasure.
 
+## Contents
+
+- [Where things are](#️-where-things-are)
+- [Structural overview](#-structural-overview)
+- [Recommended reading order](#-recommended-reading-order)
+- [Core Idea](#core-idea)
+- [Definition (Foundation)](#definition-foundation)
+- [Canonical identity layer (2026-05-27)](#canonical-identity-layer-2026-05-27)
+- [Current Status Snapshot](#current-status-snapshot)
+  - [Echo Core (load-bearing for the identity claim)](#echo-core-load-bearing-for-the-identity-claim)
+  - [Ordinal / Buchholz (parallel-independent, experimental)](#ordinal--buchholz-parallel-independent-experimental)
+- [External Bridge Targets (local workspace)](#external-bridge-targets-local-workspace)
+- [What Echo Types Are For](#what-echo-types-are-for)
+- [Identity Claim and Falsifiability](#identity-claim-and-falsifiability)
+- [Build](#build)
+  - [Installing as a library](#installing-as-a-library)
+- [Citation](#citation)
+- [Tooling Stance](#tooling-stance)
+- [Roadmap](#roadmap)
+- [Licensing](#licensing)
+
 ## 🗺️ Where things are
 
 **New here, or can't find something? Start with the
@@ -446,6 +467,5 @@ into `roadmap.adoc` and removed.
 
 ## Licensing
 
-- code, proofs, and tooling in this repository are intended to be licensed under `MPL-2.0`; see [LICENSE](LICENSE)
-- prose documentation in `docs/`, `README.md`, `readme.adoc`, and roadmap files is intended to be licensed under `CC-BY-4.0`; see [LICENSE-docs](LICENSE-docs)
-- current practical follow-up: normalize per-file SPDX headers across source files so file-level notices match the top-level licensing intent
+- code, proofs, and tooling in this repository are licensed under `MPL-2.0`; see [LICENSE](LICENSE)
+- prose documentation in `docs/`, `README.md`, `readme.adoc`, and roadmap files is licensed under `CC-BY-4.0`; see [LICENSE-docs](LICENSE-docs)

readme.adoc

@@ -1,6 +1,8 @@
 // SPDX-License-Identifier: CC-BY-4.0
 // SPDX-FileCopyrightText: 2025-2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 = echo-types
+:toc:
+:toclevels: 2
 
 image::docs/assets/banner.png[Echo Types — Proof-Relevant Information Residues,align="center"]
 
@@ -284,6 +286,5 @@ Open/gated work and cross-repo follow-ups are tracked in:
 
 == Licensing
 
-* code, proofs, and tooling in this repository are intended to be licensed under `MPL-2.0`; see `LICENSE`
-* prose documentation in `docs/`, `README.md`, `readme.adoc`, and roadmap files is intended to be licensed under `CC-BY-4.0`; see `LICENSE-docs`
-* practical follow-up: normalize per-file SPDX headers across source files so file-level notices match the top-level licensing intent
+* code, proofs, and tooling in this repository are licensed under `MPL-2.0`; see `LICENSE`
+* prose documentation in `docs/`, `README.md`, `readme.adoc`, and roadmap files is licensed under `CC-BY-4.0`; see `LICENSE-docs`