feat(cat5): break out isolates by entity_type

[jphein]

Summary

Closes #14.

• Adds isolated_by_type: dict[str, int] to GapDetectionReport — counts isolated nodes grouped by entity_type
• Computed during the existing component iteration, zero additional graph traversal
• format_report() renders the breakdown when isolates exist (e.g., topic: 1)

Test plan

• test_isolated_by_type — verifies the dict matches fixture ground truth
• test_format_report_shows_isolate_types — verifies rendered output contains type labels
• Existing Cat 5 tests pass unchanged

🫏 Generated with Claude Code

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[M0nkeyFl0wer]

Ship. Isolate-by-entity-type is the right diagnostic surface — aggregate isolate count hides which entity classes the ingestion pipeline is dropping on the floor, and computing it during the existing component iteration is the zero-cost shape. Tests verify both population and formatting. Clean.