docs(README): add == The four layers section per PRESERVATION-DESIGN §12.2 + L4 truth fix

[hyperpolymath]

Summary

Executes formal/PRESERVATION-DESIGN.md §12.2 steps 1-4 verbatim, plus fixes a stale truth claim on line 66.

What changed

1. == The four layers — new section between == What this is and == Three superpowers, listing L1-L4 in a [cols="1,3"] table. Each row points to the proof artefact that backs the layer's claim.
2. "Dyadic discipline" row — labelled (L2) and cross-references the new section.
3. "Region-based memory" row — replaced with the L1 framing (capability threading + sibling region-exit invariant).
4. "Soundness story" footnote — added, linking the four-layer separation to the verified counterexample (formal/Counterexample.v).

Also fixes a stale truth claim: line 66 said "L4 (dyadic mode) not started" — but formal/L4.v has carried the ProgramMode + program_mode_to_modality scaffold for several cycles. Updated to "mechanical scaffold landed (definitions only by design; no proof obligations)."

Source authority

• formal/PRESERVATION-DESIGN.md §12.2 — the rollout spec (verbatim draft text used).
• formal/L4-DYADIC.md — prescribes L4 as definitions-only by design (hence the "no proof obligations" claim in the README table).

Scope

Pure documentation. No proof, code, workflow, or schema changes. CI gates that don't apply to README-only changes are expected to remain green or untouched.

What this is NOT

Companion §12.3-12.7 edits to EXPLAINME.adoc / ROADMAP.adoc / SPEC.md / EPHAPAX-VISION.adoc / ephapax-linear/README.md — those are separate PRs (the §12.2 rollout spec deliberately splits them per-document so each can be reviewed in isolation).

Test plan

• AsciiDoc renders without warnings (verified via asciidoctor skipped — not installed locally; structure is identical to the existing tables so syntactic validity is preserved by construction).
• GPG signature on commit (gpg: Good signature — key 9639451754496E51D6B537CAD119017EBF695AB1).
• All claims in the new section are backed by the cited proof artefact (TypingL1.v / Modality.v / Echo.v / L4.v).

🤖 Generated with Claude Code

[github-actions]

🔍 Hypatia Security Scan

Findings: 73 issues detected

Severity	Count
🔴 Critical	9
🟠 High	16
🟡 Medium	48

⚠️ Action Required: Critical security issues found!

View findings

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