Support parallelism on the test command

[jviotti]

Signed-off-by: Juan Cruz Viotti jv@jviotti.com

[augmentcode]

🤖 Augment PR Summary

Summary: This PR adds configurable parallel execution to the jsonschema test command.

Changes:

• Enables Core's parallel module and links it into the CLI target.
• Adds --jobs/-J <count> to CLI parsing, Bash/Zsh completion, help, and test documentation.
• Defaults job selection to the system hardware-concurrency value and validates explicit job counts.
• Pre-warms configuration and resolver caches before dispatching test suites to workers.
• Refactors text and CTRF reporting into per-suite execution helpers.
• Aggregates CTRF suite reports after worker completion to retain deterministic report assembly.
• Captures worker exceptions and rethrows the first observed failure after parallel processing.
• Adds coverage for invalid job values, multi-job pass/fail cases, JSON reports, empty suites, configurations, and parse errors.

Technical Notes: Parallel text output is synchronized to avoid interleaved suite reports; documentation notes ordering, memory, and diagnostic-output considerations.

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[augmentcode]

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