feat(test): add compatibility report generator

[chaliy]

Summary

Add comprehensive compatibility report that shows per-category statistics and overall bash compatibility score.

Changes

• Add compatibility_report test in spec_tests.rs
• Add just compat-report command
• Update specs/004-testing.md with documentation

Example Output

╔══════════════════════════════════════════════════════════════════╗
║                 BashKit Compatibility Report                     ║
╚══════════════════════════════════════════════════════════════════╝

┌─────────────┬───────┬────────┬─────────┬───────────┬─────────────────┐
│  Category   │ Total │ Passed │ Skipped │ BashDiff  │   Bash Compat   │
├─────────────┼───────┼────────┼─────────┼───────────┼─────────────────┤
│    bash     │  404  │ 294/294│   110   │    21     │ 273/273 (100.0%)│
│     awk     │  89   │  48/48 │   41    │     0     │  48/48  (100.0%)│
│    grep     │  55   │  34/34 │   21    │     0     │  33/34  ( 97.1%)│
│     sed     │  65   │  40/40 │   25    │     0     │  40/40  (100.0%)│
│     jq      │  95   │  58/58 │   37    │     0     │  58/58  (100.0%)│
└─────────────┴───────┴────────┴─────────┴───────────┴─────────────────┘

Summary:
  Bash compatibility: 452/453 (99.8%)

Test plan

• just compat-report produces expected output
• All spec tests pass
• Clippy passes
• CI passes

https://claude.ai/code/session_01CFh6WZw1QDSGhTnCUW57qi

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