Add versioned JSON output to core commands

[konojunya]

Summary

• add --json to stack check, stack fmt, and stack render while preserving the existing human-readable output
• expose a stable versioned envelope for outcomes, exit statuses, diagnostics, operational errors, and produced artifacts
• publish the immutable v1 JSON Schema, golden fixtures, compatibility checks, consumer example, and command documentation
• keep inline formatter and renderer payloads in artifacts while representing file outputs with paths
• normalize corrupted gzip errors at the release-verification boundary so invalid archives fail closed without a traceback

Contract

• every machine-readable result writes exactly one JSON document plus a newline to stdout
• schema version 1 uses a versioned $schema URL and rejects unknown fields or incoherent outcome/exit/error combinations
• diagnostics use stable codes and severities with end-exclusive, zero-based UTF-8 byte offsets plus one-based Unicode scalar line/column positions
• argument, I/O, configuration, engine, and internal failures remain parseable and retain exit status 2; Stack source failures retain exit status 1
• help and all invocations without --json retain their current stream and exit behavior

Verification

• cargo fmt --all --check
• cargo test --locked
• cargo clippy --all-targets --all-features --locked -- -D warnings
• RUSTDOCFLAGS='-D warnings' cargo doc --no-deps --locked
• python3 scripts/generate_cli_assets.py --binary target/debug/stack --check
• release supply-chain validation (38 Node tests and 12 Python tests)
• npm ci --ignore-scripts --no-audit
• npm audit --audit-level=moderate (0 vulnerabilities)
• npm run test:cli-output-schema (7 fixtures and 3 compatibility tests)
• STACK_BINARY=target/debug/stack npm run test:consumer-prototype
• cargo llvm-cov --all-features --locked --fail-under-lines 90 --fail-under-functions 95 --fail-under-regions 90 (90.74% lines, 96.92% functions, 91.45% regions)
• cargo audit (no known RustSec vulnerabilities)
• PR CI on latest stable and Rust 1.85

Ajv 8.20.0 is pinned as a development-only schema validator, installed without lifecycle scripts, and its MIT license is recorded with the other third-party licenses.