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

crates/relicario-core/ARCHITECTURE.md

@@ -1,5 +1,7 @@
 # Architecture: relicario-core
 
+> **Audience:** contributors editing or extending `relicario-core`. This doc owns the module map for this crate, module-level invariants (e.g., no filesystem, no network), key flows at the module level, and the crate's test architecture. **Does NOT own:** crypto primitives or threat model (see [../../docs/CRYPTO.md](../../docs/CRYPTO.md), [../../docs/SECURITY.md](../../docs/SECURITY.md)), wire formats (see [../../docs/FORMATS.md](../../docs/FORMATS.md)).
+
 ## What this crate is for
 
 `relicario-core` is the platform-agnostic cryptographic and data-model heart of the
@@ -544,3 +546,7 @@ round-trip, and the oversized-image-header rejection path.
   source in this crate** (`time.rs:6-8`). Tests that need determinism pass an
   explicit `now: i64` to `prune_history` (`item.rs:219`) and similar — they
   do not stub `now_unix`.
+
+---
+
+**Next:** [../relicario-cli/ARCHITECTURE.md](../relicario-cli/ARCHITECTURE.md) — how the CLI wraps the core.