fix(core): Emit structured warning for skipped zero-baseline metrics

[khatayondev]

Description

This PR resolves Issue #13 where metrics with a 0.0 baseline were being silently skipped during comparison, resulting in them being completely missing from the final JSON report and console summary.

What was changed?

• Created a new SkippedMetric data class model to explicitly track skipped metrics.
• Updated the ComparisonResult to include a skipped list.
• Modified the compare_snapshots logic in flameiq/core/comparator.py to catch ComparisonError exceptions (thrown by compute_change_percent) and append the skipped metric to the skipped list with a detailed reason, rather than just ignoring it.
• Updated to_dict() so the skipped metrics are included in the serialized JSON output.
• Updated the console summary to explicitly state if metrics were skipped (e.g. All 2 metric(s) within threshold. (1 skipped)).

Testing

• ✅ Added a new unit test test_zero_baseline_skipped_metrics to tests/unit/test_core/test_comparator.py.
• ✅ All 145 tests pass locally via make test.

Closes #13