ci(verify): automate downstream consumer verification fan-out (network_system, pacs_system)

[kcenon]

What

Automate the downstream consumer verification path that #693 documented as a manual procedure.

Part of kcenon/common_system#684. Follow-up to #693.

Why

#693 defined the verification gate matrix in docs/contributing/VERIFICATION_GATES.md and documented downstream verification for network_system and pacs_system as manual commands (gh workflow run sanitizers.yml, cmake --preset asan/tsan/ubsan). The gate matrix is observable but the downstream fan-out is not triggered automatically by thread_system changes.

How

Approach

1. Add a workflow_dispatch (or scheduled matrix) job in thread_system that triggers sanitizers.yml and integration-tests.yml in network_system and pacs_system.
2. Surface the downstream results so a thread_system release PR can reference them.
3. Update docs/contributing/VERIFICATION_GATES.md "Missing Gates / Follow-up" section once automated.

Acceptance Criteria

• Downstream verification can be triggered without manual per-repo commands
• VERIFICATION_GATES.md reflects the automated path
• Release checklist references the automated downstream result