docs: document strict JSON consumer validation with Node

[codeforester]

Goal

Show downstream CLI authors how to verify that base-cli JSON and NDJSON output is accepted by a strict cross-language parser.

Background

Python's JSON loader accepts some non-standard numeric tokens by default. The public contract is intended for consumers such as Node JSON.parse, so the validation boundary should be visible to newcomers.

Scope

• Add a focused documentation example using Node's built-in JSON.parse or the repository's existing contract validator.
• Explain the difference between one JSON envelope and newline-delimited records.
• Point to the canonical output and JSON contract references.
• Include the expected failure shape for invalid output without embedding a real secret or private path.

Acceptance criteria

• The example uses a strict parser rather than only Python round-trip loading.
• It explains how a consumer should handle parse failure.
• It links to the existing contract fixture or validator.
• It does not redefine the public serialization contract.
• A contributor can validate the example without private infrastructure.

Validation

• Run the documented Node command or existing contract validator.
• Run git diff --check.
• Verify all internal links.

Non-goals

• Do not change serializer behavior.
• Do not add a new parser dependency.
• Do not use a secret-shaped fixture in the repository.