Emit machine-readable support-matrix from netengine validate

[aidankhogg]

Motivation

• Provide a CI-friendly, machine-readable report of active spec feature-states so automation can archive results and fail on unsupported active fields via a non-zero exit code.
• Reuse existing feature-state discovery logic to avoid duplicating validation semantics while enabling JSON serialization for tool consumption.

Description

• Add a --format text|json option to netengine validate and implement JSON output that includes each active feature-state entry with path, state, stage, reason, current_value, and default_value using _resolve_feature_state_paths and _is_active_feature_value for discovery.
• Defer enforcement of feature-state gates during initial spec load by adding validate_feature_states flags to load_spec, load_spec_with_composition, load_spec_with_environment, and the CLI helper _load_spec_for_cli, allowing the CLI to produce machine-readable results before failing.
• Add JSON-friendly helpers _jsonable_feature_value and _active_feature_state_results in netengine/cli/main.py and make the validate command emit structured payloads and exit non-zero when any active field is unsupported.
• Add tests that cover text and JSON output behavior and failure semantics in tests/test_cli.py, and update docs/support-matrix.md and docs/spec-reference.md with CI usage examples (netengine validate --format json).

Testing

• Ran unit tests with pytest -q tests/test_cli.py tests/test_feature_states.py, and all tests passed (30 passed).
• The new JSON behaviors are exercised by two added tests: test_validate_json_reports_active_feature_states and test_validate_json_unsupported_active_feature_exits_nonzero in tests/test_cli.py.

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Codex Task