docs: organize Stage 06 persistence postures

[roackb2]

Why

A new adopter can use Heddle's default files, but the onboarding path did not
prove the operational promise: one persistent host, paired session/archive
recovery, and whole-state-root backup and restore.

The examples also exposed a ladder-shape problem. Local JSON and PostgreSQL are
alternative persistence postures within one customization stage, but two
top-level 06-* directories made them look like duplicate sixth steps.

What

• add a runnable local JSON recovery proof with no model credential
• recover the complete session, archive manifest, and rolling summary through
  a fresh engine
• snapshot and restore an isolated complete state root
• group Stage 06 under examples/sdk/06-persistence/
  • local-json/ for one durable host using Heddle defaults
  • postgres-drizzle/ for a host-owned shared PostgreSQL adapter
• add a Stage 06 chooser that makes the alternatives and host responsibilities
  explicit
• update scripts, Drizzle configuration, source imports, guides, and package
  paths for the new layout
• preserve the existing package boundary: the local verifier ships, while the
  PostgreSQL reference remains excluded host-owned example code

This does not add a storage abstraction, official database adapter, or durable
in-flight execution promise.

Verification

• HEDDLE_EXAMPLE_STATE_ROOT=/private/tmp/heddle-pr258-local-json.LprgJN yarn example:local-json-storage:verify
• isolated PostgreSQL 17: migration plus yarn example:postgres-storage:verify
• yarn test: 122 unit files / 719 tests and 34 integration files / 310 tests
• yarn typecheck
• yarn lint
• yarn build
• npm dry-run manifest: new local JSON path included; PostgreSQL reference and
  both old paths excluded
• relative-link validation for all eight changed Markdown entry points
• git diff --check