Add Heller state Avro LOM diagram

[mdheller]

Summary

Adds the missing Heller state snapshot Avro LOM DOT diagram and registers it for exact renderer-output drift checking.

Changes

• Add docs/diagrams/heller_state_snapshot_lom.dot generated from fixtures/descriptors/heller/v1/heller_state_snapshot.avsc with tools/render_avro_schema_lom.py --max-depth 1.
• Register the state snapshot schema/diagram pair in tools/check_avro_lom_diagrams.py.

Validation

Connector-side validation:

• Branch is 2 commits ahead of main and 0 behind.
• Changed files are limited to docs/diagrams/heller_state_snapshot_lom.dot and tools/check_avro_lom_diagrams.py.

CI should run make verify, including exact Avro LOM drift checks for both Heller event and state diagrams.