[Enhancement] Add snapshot diffing to CostReport::assert_snapshot with human-readable diff output

[ekelemepraise-code]

Summary

cost.assert_snapshot("name") currently fails with a raw numeric comparison error when costs regress:

assertion failed: snapshot 'heavy_computation_cost' cost regressed
  left:  5123456
  right: 4987654

This doesn't help the developer understand which instruction type increased or by how much.

Proposed Output

COST REGRESSION: heavy_computation_cost
┌─────────────────────┬────────────┬────────────┬──────────┐
│ Metric              │ Baseline   │ Current    │ Delta    │
├─────────────────────┼────────────┼────────────┼──────────┤
│ Instructions        │  4,987,654 │  5,123,456 │ +2.72% ⚠│
│ Memory (bytes)      │     98,304 │     98,304 │ +0.00%   │
│ Fee (stroops)       │      1,245 │      1,278 │ +2.65% ⚠│
└─────────────────────┴────────────┴────────────┴──────────┘
Threshold: 5.00% | Status: EXCEEDED on [Instructions, Fee]

Implementation Notes

• Extend CostReport to store per-metric breakdowns
• Snapshot file format should be JSON for easy parsing
• Diff table should be printed to stderr on failure

Acceptance Criteria

• Human-readable diff table emitted on regression
• Snapshot format is stable JSON
• Tolerance can be set per-metric, not just globally
• Backwards compatible with existing snapshot files