var run --json: emit SpecResults to stdout

[AndreasKoestler]

Follow-up to #66 (optional item, split out of the docs-only PR #80).

var run today prints human-readable ✓/✗ output only. A --json flag would stream the same SpecResults records the vitest reporter persists to .var/<spec>.json — one JSON object per spec, or a single top-level array — so consumers don't need to know the on-disk layout.

The format itself is now documented and versioned: Run results reference.

Consumers (from #66 discussion)

• Supervising agents / CI gates — answer "which examples ran, which passed, what drifted, where exactly did it fail" without scraping test-runner text output
• Compliance / attestation pipelines (e.g. EU CRA evidence, Kosli-style artifact attestation) — attach "these documented behaviours were verified for this artifact" mechanically; the sourceHash fingerprint pins which version of the documented behaviour was verified

Design notes

• Shape should be exactly the documented SpecResults (version: 1, specPath, sourceHash, examples[]) — no parallel wire format
• Stability policy from the reference page applies: additive optional fields without a version bump; consumers ignore unknown fields
• var lint --json already exists but emits plan diagnostics — keep the flag semantics distinct (run results, not lint output)
• Open question: stream vs. array, and whether drift information (from var.lock.json reconciliation) is included or kept separate