[P3][cli] Add an export command (JSON dump) as the import counterpart

[cevheri]

Summary

Roadmap item (named in the closed backup-docs issue #42 as a future step: "a small export <file.json> command mirroring import" — the docs half shipped in v0.2.0; the command did not). The CLI can bulk-load (import) but cannot dump, which reads as a one-way door.

Scope

• libredb export <path> <file.json>: the kv layer as a JSON object of string values — the exact shape import consumes, so export -> import round-trips.
• Opens through readonlyFileSystem() like every read command (no lock, no writes); pinned by the same never-mutates test.
• Decide the story for non-kv namespaces honestly: v1 exports the kv layer only (documents/tables are reachable via their prefixes as raw entries — state this in the doc), or add per-lens export later.
• Well-formedness: values are already validated on write since v0.2.0; export must not re-introduce lone surrogates (JSON.stringify handles escaping).

Acceptance criteria

• export -> import round-trip test on a mixed-namespace fixture.
• Read-only guarantee pinned (byte-identical file, no lock file created).
• docs/CLI.md updated (command + backup section cross-link).
• Changeset (new user-facing command); gate green.