docs: add scope headers + Next: footers to all tour docs

Each of the eight tour docs (README, DESIGN, docs/CRYPTO,
docs/FORMATS, docs/SECURITY, crates/relicario-core/ARCHITECTURE,
crates/relicario-cli/ARCHITECTURE, extension/ARCHITECTURE) now
declares its scope in a blockquote under its H1 and ends with a
single-line "Next:" pointer to the next doc in the canonical
reading order: README → DESIGN → CRYPTO → FORMATS → SECURITY →
core → cli → extension.

Also trimmed README's mid-section "Architecture" stub to a one-
paragraph pointer at DESIGN.md (was duplicating cross-codebase
content and referencing a non-existent docs/architecture/ tree).

Renamed docs/CRYPTO.md's H1 from "Relicario — Architecture" to
"Relicario — Crypto Pipeline" to match the file's renamed scope.

Spec: docs/superpowers/specs/2026-05-30-doc-structure-redesign-design.md

docs/CRYPTO.md

@@ -1,4 +1,6 @@
-# Relicario — Architecture
+# Relicario — Crypto Pipeline
+
+> **Audience:** anyone evaluating or auditing the crypto. This doc owns Argon2id parameters and rationale, XChaCha20-Poly1305 rationale, vault creation/unlock flow diagrams, DCT-steganography embed and extract flows, and the high-level encrypted-file-format diagram. **Does NOT own:** byte-level schemas or JSON shapes (see [FORMATS.md](FORMATS.md)), attacker scenarios (see [SECURITY.md](SECURITY.md)), or per-module crypto implementation (see [../crates/relicario-core/ARCHITECTURE.md](../crates/relicario-core/ARCHITECTURE.md)).
 
 ## System Overview
 
@@ -274,3 +276,7 @@ Stolen device:               ████░░░░░░░░░░░░░
 
 Both factors compromised:    game over (same as every password manager)
 ```
+
+---
+
+**Next:** [FORMATS.md](FORMATS.md) — the byte-level wire formats.