docs(spec): add four-layer intro paragraph per PRESERVATION-DESIGN §12.6 (#261)

## Summary

Executes [`formal/PRESERVATION-DESIGN.md`
§12.6](../blob/main/formal/PRESERVATION-DESIGN.md) — the canonical-spec
rollout.

### What changed

One-paragraph intro added to both spec documents:

- `spec/SPEC.md` (formal versioned spec) — inserted after §1.1 Overview.
- `spec/ephapax-spec.md` (complete-language spec v0.1) — inserted after
the Purpose block.

The paragraph:
1. Names the four orthogonal disciplines (L1 / L2 / L3 / L4).
2. Explicitly labels the normative scope (L1 + L2) vs forward-looking
scope (L3 + L4) — directly from the §12.6 spec.
3. Cross-references `EPHAPAX-VISION.adoc` and `PRESERVATION-DESIGN.md`.

### Out of scope (deliberately)

§12.6's bullet list also calls for:
- Full L1 sibling-safety **theorem** statement in the spec.
- Marking L3 (Echo) as "future extension; not normative" in a dedicated
subsection.

Those are substantive spec edits that should each be reviewed
individually per the per-document rollout pattern §12 establishes.
Filing them as separate follow-up PRs.

## Companion PRs

- #256 — README §12.2
- #257 — EXPLAINME §12.3
- #258 — vision §12.4
- #259 — ROADMAP §12.5
- #260 — ephapax-linear §12.7

## Test plan

- [x] GPG-signed commit.
- [x] Both spec files get identical intro paragraph (verified by diff).
- [x] Cross-references point to existing files in this repo.

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

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

Author: hyperpolymath

spec/SPEC.md

@@ -13,6 +13,8 @@
 
 Ephapax (from Greek ἐφάπαξ, "once for all") is a programming language with a linear type system designed for safe memory management targeting WebAssembly.
 
+Ephapax's type system has four orthogonal disciplines: **L1** region capabilities, **L2** structural modality (linear/affine), **L3** irreversibility residue (Echo Types — planned), and **L4** dyadic interaction mode (project-level declaration). This specification is normative for L1 and L2 and forward-looking for L3 and L4. See [`docs/vision/EPHAPAX-VISION.adoc`](../docs/vision/EPHAPAX-VISION.adoc) for the dyad framing and [`formal/PRESERVATION-DESIGN.md`](../formal/PRESERVATION-DESIGN.md) for the per-layer design.
+
 ### 1.2 Design Goals
 
 1. **Memory Safety**: Prevent use-after-free, double-free, and memory leaks

spec/ephapax-spec.md

@@ -14,6 +14,8 @@ language combining:
 
 The dyadic principle: both disciplines coexist at per-binding granularity.
 
+Ephapax's type system has four orthogonal disciplines: **L1** region capabilities, **L2** structural modality (linear/affine), **L3** irreversibility residue (Echo Types — planned), and **L4** dyadic interaction mode (project-level declaration). This specification is normative for L1 and L2 and forward-looking for L3 and L4. See [`docs/vision/EPHAPAX-VISION.adoc`](../docs/vision/EPHAPAX-VISION.adoc) for the dyad framing and [`formal/PRESERVATION-DESIGN.md`](../formal/PRESERVATION-DESIGN.md) for the per-layer design.
+
 ---
 
 # PART 1: LEXICAL GRAMMAR